Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Michael Holzheu]

Am Thu, 22 Feb 2018 13:06:40 +0100
schrieb Michael Holzheu :

> Am Fri, 16 Feb 2018 21:20:09 +0530
> schrieb "Naveen N. Rao" :
> 
> > Daniel Borkmann wrote:
> > > On 02/15/2018 05:25 PM, Daniel Borkmann wrote:
> > >> On 02/13/2018 05:05 AM, Sandipan Das wrote:
> > >>> The imm field of a bpf_insn is a signed 32-bit integer. For
> > >>> JIT-ed bpf-to-bpf function calls, it stores the offset from
> > >>> __bpf_call_base to the start of the callee function.
> > >>>
> > >>> For some architectures, such as powerpc64, it was found that
> > >>> this offset may be as large as 64 bits because of which this
> > >>> cannot be accomodated in the imm field without truncation.
> > >>>
> > >>> To resolve this, we additionally make aux->func within each
> > >>> bpf_prog associated with the functions to point to the list
> > >>> of all function addresses determined by the verifier.
> > >>>
> > >>> We keep the value assigned to the off field of the bpf_insn
> > >>> as a way to index into aux->func and also set aux->func_cnt
> > >>> so that this can be used for performing basic upper bound
> > >>> checks for the off field.
> > >>>
> > >>> Signed-off-by: Sandipan Das 
> > >>> ---
> > >>> v2: Make aux->func point to the list of functions determined
> > >>> by the verifier rather than allocating a separate callee
> > >>> list for each function.
> > >> 
> > >> Approach looks good to me; do you know whether s390x JIT would
> > >> have similar requirement? I think one limitation that would still
> > >> need to be addressed later with such approach would be regarding the
> > >> xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
> > >> ("bpf: allow for correlation of maps and helpers in dump"). Any
> > >> ideas for this (potentially if we could use off + imm for calls,
> > >> we'd get to 48 bits, but that seems still not be enough as you say)?
> > 
> > All good points. I'm not really sure how s390x works, so I can't comment 
> > on that, but I'm copying Michael Holzheu for his consideration.
> > 
> > With the existing scheme, 48 bits won't be enough, so we rejected that 
> > approach. I can also see how this will be a problem with bpftool, but I 
> > haven't looked into it in detail. I wonder if we can annotate the output 
> > to indicate the function being referred to?
> > 
> > > 
> > > One other random thought, although I'm not sure how feasible this
> > > is for ppc64 JIT to realize ... but idea would be to have something
> > > like the below:
> > > 
> > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > index 29ca920..daa7258 100644
> > > --- a/kernel/bpf/core.c
> > > +++ b/kernel/bpf/core.c
> > > @@ -512,6 +512,11 @@ int bpf_get_kallsym(unsigned int symnum, unsigned 
> > > long *value, char *type,
> > >   return ret;
> > >  }
> > > 
> > > +void * __weak bpf_jit_image_alloc(unsigned long size)
> > > +{
> > > + return module_alloc(size);
> > > +}
> > > +
> > >  struct bpf_binary_header *
> > >  bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
> > >unsigned int alignment,
> > > @@ -525,7 +530,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 
> > > **image_ptr,
> > >* random section of illegal instructions.
> > >*/
> > >   size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
> > > - hdr = module_alloc(size);
> > > + hdr = bpf_jit_image_alloc(size);
> > >   if (hdr == NULL)
> > >   return NULL;
> > > 
> > > And ppc64 JIT could override bpf_jit_image_alloc() in a similar way
> > > like some archs would override the module_alloc() helper through a
> > > custom implementation, usually via __vmalloc_node_range(), so we
> > > could perhaps fit the range for BPF JITed images in a way that they
> > > could use the 32bit imm in the end? There are not that many progs
> > > loaded typically, so the range could be a bit narrower in such case
> > > anyway. (Not sure if this would work out though, but I thought to
> > > bring it up.)
> > 
> > That'd be a good option to consider. I don't think we want to allocate 
> > anything from the linear memory range since users could load 
> > unprivileged BPF programs and consume a lot of memory that way. I doubt 
> > if we can map vmalloc'ed memory into the 0xc0 address range, but I'm not 
> > entirely sure.
> > 
> > Michael,
> > Is the above possible? The question is if we can have BPF programs be 
> > allocated within 4GB of __bpf_call_base (which is a kernel symbol), so 
> > that calls to those programs can be encoded in a 32-bit immediate field 
> > in a BPF instruction. As an extension, we may be able to extend it to 
> > 48-bits by combining with another BPF instruction field (offset). In 
> > either case, the vmalloc'ed address range won't work.
> > 
> > The alternative is to pass the full 64-bit address of the BPF program in 
> > an auxiliary field (as proposed in this patch set) but we need to fix it 
> > up for 'bpftool' as 

Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Michael Holzheu]

Am Fri, 16 Feb 2018 21:20:09 +0530
schrieb "Naveen N. Rao" :

> Daniel Borkmann wrote:
> > On 02/15/2018 05:25 PM, Daniel Borkmann wrote:
> >> On 02/13/2018 05:05 AM, Sandipan Das wrote:
> >>> The imm field of a bpf_insn is a signed 32-bit integer. For
> >>> JIT-ed bpf-to-bpf function calls, it stores the offset from
> >>> __bpf_call_base to the start of the callee function.
> >>>
> >>> For some architectures, such as powerpc64, it was found that
> >>> this offset may be as large as 64 bits because of which this
> >>> cannot be accomodated in the imm field without truncation.
> >>>
> >>> To resolve this, we additionally make aux->func within each
> >>> bpf_prog associated with the functions to point to the list
> >>> of all function addresses determined by the verifier.
> >>>
> >>> We keep the value assigned to the off field of the bpf_insn
> >>> as a way to index into aux->func and also set aux->func_cnt
> >>> so that this can be used for performing basic upper bound
> >>> checks for the off field.
> >>>
> >>> Signed-off-by: Sandipan Das 
> >>> ---
> >>> v2: Make aux->func point to the list of functions determined
> >>> by the verifier rather than allocating a separate callee
> >>> list for each function.
> >> 
> >> Approach looks good to me; do you know whether s390x JIT would
> >> have similar requirement? I think one limitation that would still
> >> need to be addressed later with such approach would be regarding the
> >> xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
> >> ("bpf: allow for correlation of maps and helpers in dump"). Any
> >> ideas for this (potentially if we could use off + imm for calls,
> >> we'd get to 48 bits, but that seems still not be enough as you say)?
> 
> All good points. I'm not really sure how s390x works, so I can't comment 
> on that, but I'm copying Michael Holzheu for his consideration.
> 
> With the existing scheme, 48 bits won't be enough, so we rejected that 
> approach. I can also see how this will be a problem with bpftool, but I 
> haven't looked into it in detail. I wonder if we can annotate the output 
> to indicate the function being referred to?
> 
> > 
> > One other random thought, although I'm not sure how feasible this
> > is for ppc64 JIT to realize ... but idea would be to have something
> > like the below:
> > 
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 29ca920..daa7258 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -512,6 +512,11 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long 
> > *value, char *type,
> > return ret;
> >  }
> > 
> > +void * __weak bpf_jit_image_alloc(unsigned long size)
> > +{
> > +   return module_alloc(size);
> > +}
> > +
> >  struct bpf_binary_header *
> >  bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
> >  unsigned int alignment,
> > @@ -525,7 +530,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 
> > **image_ptr,
> >  * random section of illegal instructions.
> >  */
> > size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
> > -   hdr = module_alloc(size);
> > +   hdr = bpf_jit_image_alloc(size);
> > if (hdr == NULL)
> > return NULL;
> > 
> > And ppc64 JIT could override bpf_jit_image_alloc() in a similar way
> > like some archs would override the module_alloc() helper through a
> > custom implementation, usually via __vmalloc_node_range(), so we
> > could perhaps fit the range for BPF JITed images in a way that they
> > could use the 32bit imm in the end? There are not that many progs
> > loaded typically, so the range could be a bit narrower in such case
> > anyway. (Not sure if this would work out though, but I thought to
> > bring it up.)
> 
> That'd be a good option to consider. I don't think we want to allocate 
> anything from the linear memory range since users could load 
> unprivileged BPF programs and consume a lot of memory that way. I doubt 
> if we can map vmalloc'ed memory into the 0xc0 address range, but I'm not 
> entirely sure.
> 
> Michael,
> Is the above possible? The question is if we can have BPF programs be 
> allocated within 4GB of __bpf_call_base (which is a kernel symbol), so 
> that calls to those programs can be encoded in a 32-bit immediate field 
> in a BPF instruction. As an extension, we may be able to extend it to 
> 48-bits by combining with another BPF instruction field (offset). In 
> either case, the vmalloc'ed address range won't work.
> 
> The alternative is to pass the full 64-bit address of the BPF program in 
> an auxiliary field (as proposed in this patch set) but we need to fix it 
> up for 'bpftool' as well.

Hi Naveen,

Our s390 kernel maintainer Martin Schwidefsky took over
eBPF responsibility for s390 from me.

@Martin: Can you answer Navee's question?

Michael

Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Naveen N. Rao]

Michael Ellerman wrote:

"Naveen N. Rao"  writes:

Daniel Borkmann wrote:

On 02/15/2018 05:25 PM, Daniel Borkmann wrote:

On 02/13/2018 05:05 AM, Sandipan Das wrote:

The imm field of a bpf_insn is a signed 32-bit integer. For
JIT-ed bpf-to-bpf function calls, it stores the offset from
__bpf_call_base to the start of the callee function.

For some architectures, such as powerpc64, it was found that
this offset may be as large as 64 bits because of which this
cannot be accomodated in the imm field without truncation.

To resolve this, we additionally make aux->func within each
bpf_prog associated with the functions to point to the list
of all function addresses determined by the verifier.

We keep the value assigned to the off field of the bpf_insn
as a way to index into aux->func and also set aux->func_cnt
so that this can be used for performing basic upper bound
checks for the off field.

Signed-off-by: Sandipan Das 
---
v2: Make aux->func point to the list of functions determined
by the verifier rather than allocating a separate callee
list for each function.


Approach looks good to me; do you know whether s390x JIT would
have similar requirement? I think one limitation that would still
need to be addressed later with such approach would be regarding the
xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
("bpf: allow for correlation of maps and helpers in dump"). Any
ideas for this (potentially if we could use off + imm for calls,
we'd get to 48 bits, but that seems still not be enough as you say)?


All good points. I'm not really sure how s390x works, so I can't comment 
on that, but I'm copying Michael Holzheu for his consideration.


With the existing scheme, 48 bits won't be enough, so we rejected that 
approach. I can also see how this will be a problem with bpftool, but I 
haven't looked into it in detail. I wonder if we can annotate the output 
to indicate the function being referred to?




One other random thought, although I'm not sure how feasible this
is for ppc64 JIT to realize ... but idea would be to have something
like the below:

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 29ca920..daa7258 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -512,6 +512,11 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long 
*value, char *type,
return ret;
 }

+void * __weak bpf_jit_image_alloc(unsigned long size)
+{
+   return module_alloc(size);
+}
+
 struct bpf_binary_header *
 bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 unsigned int alignment,
@@ -525,7 +530,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 * random section of illegal instructions.
 */
size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
-   hdr = module_alloc(size);
+   hdr = bpf_jit_image_alloc(size);
if (hdr == NULL)
return NULL;

And ppc64 JIT could override bpf_jit_image_alloc() in a similar way
like some archs would override the module_alloc() helper through a
custom implementation, usually via __vmalloc_node_range(), so we
could perhaps fit the range for BPF JITed images in a way that they
could use the 32bit imm in the end? There are not that many progs
loaded typically, so the range could be a bit narrower in such case
anyway. (Not sure if this would work out though, but I thought to
bring it up.)


That'd be a good option to consider. I don't think we want to allocate 
anything from the linear memory range since users could load 
unprivileged BPF programs and consume a lot of memory that way. I doubt 
if we can map vmalloc'ed memory into the 0xc0 address range, but I'm not 
entirely sure.


Michael,
Is the above possible? The question is if we can have BPF programs be 
allocated within 4GB of __bpf_call_base (which is a kernel symbol), so 
that calls to those programs can be encoded in a 32-bit immediate field 
in a BPF instruction.


Hmmm.

It's not technically impossible, but I don't think it's really a good
option.

The 0xc range is a linear mapping of RAM, and the kernel tends to be
near the start of RAM for reasons. That means there generally isn't a
hole in the 0xc range within 4GB for you to map BPF programs.

You could create a hole by making the 0xc mapping non linear, ie.
mapping some RAM near the kernel elsewhere in the 0xc range, to make a
hole that you can then remap BPF programs into. But I think that would
cause a lot of bugs, it's a pretty fundamental assumption that the
linear mapping is 1:1.

As an extension, we may be able to extend it to 
48-bits by combining with another BPF instruction field (offset). In 
either case, the vmalloc'ed address range won't work.


48-bits could possibly work, we don't have systems with that much RAM
*yet*. So you could remap the BPF programs at the end of the 0xc range,
or somewhere we have a gap in RAM.

That would probably still 

Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Michael Ellerman]

"Naveen N. Rao"  writes:
> Daniel Borkmann wrote:
>> On 02/15/2018 05:25 PM, Daniel Borkmann wrote:
>>> On 02/13/2018 05:05 AM, Sandipan Das wrote:
 The imm field of a bpf_insn is a signed 32-bit integer. For
 JIT-ed bpf-to-bpf function calls, it stores the offset from
 __bpf_call_base to the start of the callee function.

 For some architectures, such as powerpc64, it was found that
 this offset may be as large as 64 bits because of which this
 cannot be accomodated in the imm field without truncation.

 To resolve this, we additionally make aux->func within each
 bpf_prog associated with the functions to point to the list
 of all function addresses determined by the verifier.

 We keep the value assigned to the off field of the bpf_insn
 as a way to index into aux->func and also set aux->func_cnt
 so that this can be used for performing basic upper bound
 checks for the off field.

 Signed-off-by: Sandipan Das 
 ---
 v2: Make aux->func point to the list of functions determined
 by the verifier rather than allocating a separate callee
 list for each function.
>>> 
>>> Approach looks good to me; do you know whether s390x JIT would
>>> have similar requirement? I think one limitation that would still
>>> need to be addressed later with such approach would be regarding the
>>> xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
>>> ("bpf: allow for correlation of maps and helpers in dump"). Any
>>> ideas for this (potentially if we could use off + imm for calls,
>>> we'd get to 48 bits, but that seems still not be enough as you say)?
>
> All good points. I'm not really sure how s390x works, so I can't comment 
> on that, but I'm copying Michael Holzheu for his consideration.
>
> With the existing scheme, 48 bits won't be enough, so we rejected that 
> approach. I can also see how this will be a problem with bpftool, but I 
> haven't looked into it in detail. I wonder if we can annotate the output 
> to indicate the function being referred to?
>
>> 
>> One other random thought, although I'm not sure how feasible this
>> is for ppc64 JIT to realize ... but idea would be to have something
>> like the below:
>> 
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index 29ca920..daa7258 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -512,6 +512,11 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long 
>> *value, char *type,
>>  return ret;
>>  }
>> 
>> +void * __weak bpf_jit_image_alloc(unsigned long size)
>> +{
>> +return module_alloc(size);
>> +}
>> +
>>  struct bpf_binary_header *
>>  bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>>   unsigned int alignment,
>> @@ -525,7 +530,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 
>> **image_ptr,
>>   * random section of illegal instructions.
>>   */
>>  size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
>> -hdr = module_alloc(size);
>> +hdr = bpf_jit_image_alloc(size);
>>  if (hdr == NULL)
>>  return NULL;
>> 
>> And ppc64 JIT could override bpf_jit_image_alloc() in a similar way
>> like some archs would override the module_alloc() helper through a
>> custom implementation, usually via __vmalloc_node_range(), so we
>> could perhaps fit the range for BPF JITed images in a way that they
>> could use the 32bit imm in the end? There are not that many progs
>> loaded typically, so the range could be a bit narrower in such case
>> anyway. (Not sure if this would work out though, but I thought to
>> bring it up.)
>
> That'd be a good option to consider. I don't think we want to allocate 
> anything from the linear memory range since users could load 
> unprivileged BPF programs and consume a lot of memory that way. I doubt 
> if we can map vmalloc'ed memory into the 0xc0 address range, but I'm not 
> entirely sure.
>
> Michael,
> Is the above possible? The question is if we can have BPF programs be 
> allocated within 4GB of __bpf_call_base (which is a kernel symbol), so 
> that calls to those programs can be encoded in a 32-bit immediate field 
> in a BPF instruction.

Hmmm.

It's not technically impossible, but I don't think it's really a good
option.

The 0xc range is a linear mapping of RAM, and the kernel tends to be
near the start of RAM for reasons. That means there generally isn't a
hole in the 0xc range within 4GB for you to map BPF programs.

You could create a hole by making the 0xc mapping non linear, ie.
mapping some RAM near the kernel elsewhere in the 0xc range, to make a
hole that you can then remap BPF programs into. But I think that would
cause a lot of bugs, it's a pretty fundamental assumption that the
linear mapping is 1:1.

> As an extension, we may be able to extend it to 
> 48-bits by combining with another BPF instruction field (offset). In 
> either case, the 

Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Naveen N. Rao]

Daniel Borkmann wrote:

On 02/15/2018 05:25 PM, Daniel Borkmann wrote:

On 02/13/2018 05:05 AM, Sandipan Das wrote:

The imm field of a bpf_insn is a signed 32-bit integer. For
JIT-ed bpf-to-bpf function calls, it stores the offset from
__bpf_call_base to the start of the callee function.

For some architectures, such as powerpc64, it was found that
this offset may be as large as 64 bits because of which this
cannot be accomodated in the imm field without truncation.

To resolve this, we additionally make aux->func within each
bpf_prog associated with the functions to point to the list
of all function addresses determined by the verifier.

We keep the value assigned to the off field of the bpf_insn
as a way to index into aux->func and also set aux->func_cnt
so that this can be used for performing basic upper bound
checks for the off field.

Signed-off-by: Sandipan Das 
---
v2: Make aux->func point to the list of functions determined
by the verifier rather than allocating a separate callee
list for each function.


Approach looks good to me; do you know whether s390x JIT would
have similar requirement? I think one limitation that would still
need to be addressed later with such approach would be regarding the
xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
("bpf: allow for correlation of maps and helpers in dump"). Any
ideas for this (potentially if we could use off + imm for calls,
we'd get to 48 bits, but that seems still not be enough as you say)?


All good points. I'm not really sure how s390x works, so I can't comment 
on that, but I'm copying Michael Holzheu for his consideration.


With the existing scheme, 48 bits won't be enough, so we rejected that 
approach. I can also see how this will be a problem with bpftool, but I 
haven't looked into it in detail. I wonder if we can annotate the output 
to indicate the function being referred to?




One other random thought, although I'm not sure how feasible this
is for ppc64 JIT to realize ... but idea would be to have something
like the below:

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 29ca920..daa7258 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -512,6 +512,11 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long 
*value, char *type,
return ret;
 }

+void * __weak bpf_jit_image_alloc(unsigned long size)
+{
+   return module_alloc(size);
+}
+
 struct bpf_binary_header *
 bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 unsigned int alignment,
@@ -525,7 +530,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 * random section of illegal instructions.
 */
size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
-   hdr = module_alloc(size);
+   hdr = bpf_jit_image_alloc(size);
if (hdr == NULL)
return NULL;

And ppc64 JIT could override bpf_jit_image_alloc() in a similar way
like some archs would override the module_alloc() helper through a
custom implementation, usually via __vmalloc_node_range(), so we
could perhaps fit the range for BPF JITed images in a way that they
could use the 32bit imm in the end? There are not that many progs
loaded typically, so the range could be a bit narrower in such case
anyway. (Not sure if this would work out though, but I thought to
bring it up.)


That'd be a good option to consider. I don't think we want to allocate 
anything from the linear memory range since users could load 
unprivileged BPF programs and consume a lot of memory that way. I doubt 
if we can map vmalloc'ed memory into the 0xc0 address range, but I'm not 
entirely sure.


Michael,
Is the above possible? The question is if we can have BPF programs be 
allocated within 4GB of __bpf_call_base (which is a kernel symbol), so 
that calls to those programs can be encoded in a 32-bit immediate field 
in a BPF instruction. As an extension, we may be able to extend it to 
48-bits by combining with another BPF instruction field (offset). In 
either case, the vmalloc'ed address range won't work.


The alternative is to pass the full 64-bit address of the BPF program in 
an auxiliary field (as proposed in this patch set) but we need to fix it 
up for 'bpftool' as well.


Thanks,
Naveen

Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Daniel Borkmann]

On 02/15/2018 05:25 PM, Daniel Borkmann wrote:
> On 02/13/2018 05:05 AM, Sandipan Das wrote:
>> The imm field of a bpf_insn is a signed 32-bit integer. For
>> JIT-ed bpf-to-bpf function calls, it stores the offset from
>> __bpf_call_base to the start of the callee function.
>>
>> For some architectures, such as powerpc64, it was found that
>> this offset may be as large as 64 bits because of which this
>> cannot be accomodated in the imm field without truncation.
>>
>> To resolve this, we additionally make aux->func within each
>> bpf_prog associated with the functions to point to the list
>> of all function addresses determined by the verifier.
>>
>> We keep the value assigned to the off field of the bpf_insn
>> as a way to index into aux->func and also set aux->func_cnt
>> so that this can be used for performing basic upper bound
>> checks for the off field.
>>
>> Signed-off-by: Sandipan Das 
>> ---
>> v2: Make aux->func point to the list of functions determined
>> by the verifier rather than allocating a separate callee
>> list for each function.
> 
> Approach looks good to me; do you know whether s390x JIT would
> have similar requirement? I think one limitation that would still
> need to be addressed later with such approach would be regarding the
> xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
> ("bpf: allow for correlation of maps and helpers in dump"). Any
> ideas for this (potentially if we could use off + imm for calls,
> we'd get to 48 bits, but that seems still not be enough as you say)?

One other random thought, although I'm not sure how feasible this
is for ppc64 JIT to realize ... but idea would be to have something
like the below:

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 29ca920..daa7258 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -512,6 +512,11 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long 
*value, char *type,
return ret;
 }

+void * __weak bpf_jit_image_alloc(unsigned long size)
+{
+   return module_alloc(size);
+}
+
 struct bpf_binary_header *
 bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 unsigned int alignment,
@@ -525,7 +530,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 * random section of illegal instructions.
 */
size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
-   hdr = module_alloc(size);
+   hdr = bpf_jit_image_alloc(size);
if (hdr == NULL)
return NULL;

And ppc64 JIT could override bpf_jit_image_alloc() in a similar way
like some archs would override the module_alloc() helper through a
custom implementation, usually via __vmalloc_node_range(), so we
could perhaps fit the range for BPF JITed images in a way that they
could use the 32bit imm in the end? There are not that many progs
loaded typically, so the range could be a bit narrower in such case
anyway. (Not sure if this would work out though, but I thought to
bring it up.)

Thanks,
Daniel

Re: [RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Daniel Borkmann]

On 02/13/2018 05:05 AM, Sandipan Das wrote:
> The imm field of a bpf_insn is a signed 32-bit integer. For
> JIT-ed bpf-to-bpf function calls, it stores the offset from
> __bpf_call_base to the start of the callee function.
> 
> For some architectures, such as powerpc64, it was found that
> this offset may be as large as 64 bits because of which this
> cannot be accomodated in the imm field without truncation.
> 
> To resolve this, we additionally make aux->func within each
> bpf_prog associated with the functions to point to the list
> of all function addresses determined by the verifier.
> 
> We keep the value assigned to the off field of the bpf_insn
> as a way to index into aux->func and also set aux->func_cnt
> so that this can be used for performing basic upper bound
> checks for the off field.
> 
> Signed-off-by: Sandipan Das 
> ---
> v2: Make aux->func point to the list of functions determined
> by the verifier rather than allocating a separate callee
> list for each function.

Approach looks good to me; do you know whether s390x JIT would
have similar requirement? I think one limitation that would still
need to be addressed later with such approach would be regarding the
xlated prog dump in bpftool, see 'BPF calls via JIT' in 7105e828c087
("bpf: allow for correlation of maps and helpers in dump"). Any
ideas for this (potentially if we could use off + imm for calls,
we'd get to 48 bits, but that seems still not be enough as you say)?

Thanks,
Daniel

[RFC][PATCH bpf v2 1/2] bpf: allow 64-bit offsets for bpf function calls

[Sandipan Das]

The imm field of a bpf_insn is a signed 32-bit integer. For
JIT-ed bpf-to-bpf function calls, it stores the offset from
__bpf_call_base to the start of the callee function.

For some architectures, such as powerpc64, it was found that
this offset may be as large as 64 bits because of which this
cannot be accomodated in the imm field without truncation.

To resolve this, we additionally make aux->func within each
bpf_prog associated with the functions to point to the list
of all function addresses determined by the verifier.

We keep the value assigned to the off field of the bpf_insn
as a way to index into aux->func and also set aux->func_cnt
so that this can be used for performing basic upper bound
checks for the off field.

Signed-off-by: Sandipan Das 
---
v2: Make aux->func point to the list of functions determined
by the verifier rather than allocating a separate callee
list for each function.
---
 kernel/bpf/verifier.c | 16 +++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5fb69a85d967..1c4d9cd485ed 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5288,11 +5288,25 @@ static int jit_subprogs(struct bpf_verifier_env *env)
insn->src_reg != BPF_PSEUDO_CALL)
continue;
subprog = insn->off;
-   insn->off = 0;
insn->imm = (u64 (*)(u64, u64, u64, u64, u64))
func[subprog]->bpf_func -
__bpf_call_base;
}
+
+   /* the offset to a callee function from __bpf_call_base
+* may be larger than what the 32 bit integer imm can
+* accomodate which will truncate the higher order bits
+*
+* to avoid this, we additionally utilize the aux data
+* of each function to point to a list of all function
+* addresses determined by the verifier
+*
+* the off field of the instruction provides the index
+* in this list where the start address of a function
+* is available
+*/
+   func[i]->aux->func = func;
+   func[i]->aux->func_cnt = env->subprog_cnt + 1;
}
for (i = 0; i <= env->subprog_cnt; i++) {
old_bpf_func = func[i]->bpf_func;
-- 
2.14.3