Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Sat, 29 Dec 2018 at 16:49, Andy Lutomirski wrote: > > Could you use a prctl to set whether you were running in 32 or 64 bit > > mode? Or do you change which kind of task you're emulating too often > > to make this a good idea? QEMU's linux-user mode always only runs the single process, which is a fixed guest architecture. But it also wants to make system calls on its own behalf, as well as the ones it is passing through from the guest, and I suspect it would confuse the host libc if we changed the semantics of those under its feet. > How would this work? We already have the separate > COMPAT_DEFINE_SYSCALL entries *and* in_compat_syscall(). Now we’d have > a third degree of freedom. > > Either the arches people care about should add reasonable ways to > issue 32-bit syscalls from 64-bit mode or there should be an explicit > way to ask for the 32-bit directory offsets. The first of those is not sufficient for QEMU if done as a per-architecture thing, because there may not even be a 32-bit syscall interface on the host kernel. The second sounds better -- there's nothing conceptually architecture specific about what we want to do or which is tied to the idea of whether there's a 32-bit compat mode in the host architecture or not. thanks -- PMM
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
> On Dec 28, 2018, at 6:54 PM, Matthew Wilcox wrote: > >> On Sat, Dec 29, 2018 at 12:12:27AM +, Peter Maydell wrote: >> On Fri, 28 Dec 2018 at 23:16, Andreas Dilger wrot >>> On Dec 28, 2018, at 4:18 AM, Peter Maydell wrote: The problem is that there is no 32-bit API in some cases (unless I have misunderstood the kernel code) -- not all host architectures implement compat syscalls or allow them to be called from 64-bit processes or implement all the older syscall variants that had smaller offets. If there was a guaranteed "this syscall always exists and always gives me 32-bit offsets" we could use it. >>> >>> The "32bitapi" mount option would use 32-bit hash for seekdir >>> and telldir, regardless of what kernel API was used. That would >>> just set the FMODE_32BITHASH flag in the file->f_mode for all files. >> >> A mount option wouldn't be much use to QEMU -- we can't tell >> our users how to mount their filesystems, which they're >> often doing lots of other things with besides running QEMU. >> (Otherwise we could just tell them "don't use ext4", which >> would also solve the problem :-)) We need something we can >> use at the individual-syscall level. > > Could you use a prctl to set whether you were running in 32 or 64 bit > mode? Or do you change which kind of task you're emulating too often > to make this a good idea? How would this work? We already have the separate COMPAT_DEFINE_SYSCALL entries *and* in_compat_syscall(). Now we’d have a third degree of freedom. Either the arches people care about should add reasonable ways to issue 32-bit syscalls from 64-bit mode or there should be an explicit way to ask for the 32-bit directory offsets.
Re: [V9fs-developer] [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
Theodore Y. Ts'o wrote on Fri, Dec 28, 2018: > On Sat, Dec 29, 2018 at 03:37:21AM +0100, Dominique Martinet wrote: > > > Are there going to be cases where a process or a thread will sometimes > > > want the 64-bit interface, and sometimes want the 32-bit interface? > > > Or is it always going to be one or the other? I wonder if we could > > > simply add a new flag to the process personality(2) flags. > > > > That would likely work for qemu user, but the qemu system+9p case is > > going to be more painful.. > > More precisely, the 9p protocol does not plan for anything other than > > 64bit offset so if the vfs needs to hand out a 32bit offset we'll need > > to make a correspondance table between the 32bit offsets we hand off and > > the 64bit ones to use; unless some flag can be passed at lopen to tell > > the server to always hand out 32bit offsets for this directory... And if > > we do that then 9p servers will need a way to use both APIs in parallel > > for both types of directories. > > How about if we add a fcntl(2) mediated flag, which is tied to a > struct file? Would that be more or less painful for 9p and qemu > system+9p? Hmm. 9P2000.L doesn't have anything akin to fcntl either, the only two obvious places where we could pass a flag is lopen (which already handles a bunch of linux-specific flags, e.g. passing O_LARGEFILE O_NOATIME etc will just forward these through for qemu/diod at least), or adding a new parameter to the 9p readdir. The former would let us get away without modifying the protocol as servers will just ignore flags they don't handle on implementations I checked, so it'd definitely be the least effort choice from what I can tell. On the other hand a fcntl would solve the server-side problem, it'd allow the server to request appropriately-sized offsets per fd, so it's a good start; we "just" need to figure how to translate that on the wire. -- Dominique Martinet | Asmadeus
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Sat, Dec 29, 2018 at 03:37:21AM +0100, Dominique Martinet wrote: > > Are there going to be cases where a process or a thread will sometimes > > want the 64-bit interface, and sometimes want the 32-bit interface? > > Or is it always going to be one or the other? I wonder if we could > > simply add a new flag to the process personality(2) flags. > > That would likely work for qemu user, but the qemu system+9p case is > going to be more painful.. > More precisely, the 9p protocol does not plan for anything other than > 64bit offset so if the vfs needs to hand out a 32bit offset we'll need > to make a correspondance table between the 32bit offsets we hand off and > the 64bit ones to use; unless some flag can be passed at lopen to tell > the server to always hand out 32bit offsets for this directory... And if > we do that then 9p servers will need a way to use both APIs in parallel > for both types of directories. How about if we add a fcntl(2) mediated flag, which is tied to a struct file? Would that be more or less painful for 9p and qemu system+9p? - Ted
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
Theodore Y. Ts'o wrote on Fri, Dec 28, 2018: > > The problem is that there is no 32-bit API in some cases > > (unless I have misunderstood the kernel code) -- not all > > host architectures implement compat syscalls or allow them > > to be called from 64-bit processes or implement all the older > > syscall variants that had smaller offets. If there was a guaranteed > > "this syscall always exists and always gives me 32-bit offsets" > > we could use it. > > Are there going to be cases where a process or a thread will sometimes > want the 64-bit interface, and sometimes want the 32-bit interface? > Or is it always going to be one or the other? I wonder if we could > simply add a new flag to the process personality(2) flags. That would likely work for qemu user, but the qemu system+9p case is going to be more painful.. More precisely, the 9p protocol does not plan for anything other than 64bit offset so if the vfs needs to hand out a 32bit offset we'll need to make a correspondance table between the 32bit offsets we hand off and the 64bit ones to use; unless some flag can be passed at lopen to tell the server to always hand out 32bit offsets for this directory... And if we do that then 9p servers will need a way to use both APIs in parallel for both types of directories. (Although I'd rather not have to do either in the first place, keeping track of all used offsets just in case seems like a waste even if we only do it for processes in 32bit mode, and a new flag would be a protocol change with 9p not being designed to catter for subtle protocol changes so would be rather painful to roll out) No bright idea here, sorry. -- Dominique Martinet | Asmadeus
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Fri, Dec 28, 2018 at 11:18:18AM +, Peter Maydell wrote: > In general inodes and offsets start from 0 and work up -- > so almost all of the time they don't actually overflow. > The problem with ext4 directory hash "offsets" is that they > overflow all the time and immediately, so instead of "works > unless you have a weird edge case" like all the other filesystems,h > it's "never works". Actually, XFS uses the inode number to encode the location of the inode (it doesn't have a fixed inode table, so it's effectively the block number shifted left by 3 or 4 bits, with the low bits indicating the slot in the 4k block). It has a hack to provide backwards compatibility for 32-bit API's, but there is a similar, "oh, we're on a non-paleolithic CPU, let's use the full 64-bits" sort of logic that ext4 has. > The problem is that there is no 32-bit API in some cases > (unless I have misunderstood the kernel code) -- not all > host architectures implement compat syscalls or allow them > to be called from 64-bit processes or implement all the older > syscall variants that had smaller offets. If there was a guaranteed > "this syscall always exists and always gives me 32-bit offsets" > we could use it. Are there going to be cases where a process or a thread will sometimes want the 64-bit interface, and sometimes want the 32-bit interface? Or is it always going to be one or the other? I wonder if we could simply add a new flag to the process personality(2) flags. > Yes, that has been suggested, but it seemed a bit dubious > to bake in knowledge of ext4's internal implementation details. > Can we rely on this as an ABI promise that will always work > for all versions of all file systems going forwards? Yeah, that seems dubious because I'm pretty sure there are other file systems that may have their own 32/64-bit quirks. - Ted
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Sat, Dec 29, 2018 at 12:12:27AM +, Peter Maydell wrote: > On Fri, 28 Dec 2018 at 23:16, Andreas Dilger wrot > > On Dec 28, 2018, at 4:18 AM, Peter Maydell wrote: > > > The problem is that there is no 32-bit API in some cases > > > (unless I have misunderstood the kernel code) -- not all > > > host architectures implement compat syscalls or allow them > > > to be called from 64-bit processes or implement all the older > > > syscall variants that had smaller offets. If there was a guaranteed > > > "this syscall always exists and always gives me 32-bit offsets" > > > we could use it. > > > > The "32bitapi" mount option would use 32-bit hash for seekdir > > and telldir, regardless of what kernel API was used. That would > > just set the FMODE_32BITHASH flag in the file->f_mode for all files. > > A mount option wouldn't be much use to QEMU -- we can't tell > our users how to mount their filesystems, which they're > often doing lots of other things with besides running QEMU. > (Otherwise we could just tell them "don't use ext4", which > would also solve the problem :-)) We need something we can > use at the individual-syscall level. Could you use a prctl to set whether you were running in 32 or 64 bit mode? Or do you change which kind of task you're emulating too often to make this a good idea?
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Fri, 28 Dec 2018 at 23:16, Andreas Dilger wrot > On Dec 28, 2018, at 4:18 AM, Peter Maydell wrote: > > The problem is that there is no 32-bit API in some cases > > (unless I have misunderstood the kernel code) -- not all > > host architectures implement compat syscalls or allow them > > to be called from 64-bit processes or implement all the older > > syscall variants that had smaller offets. If there was a guaranteed > > "this syscall always exists and always gives me 32-bit offsets" > > we could use it. > > The "32bitapi" mount option would use 32-bit hash for seekdir > and telldir, regardless of what kernel API was used. That would > just set the FMODE_32BITHASH flag in the file->f_mode for all files. A mount option wouldn't be much use to QEMU -- we can't tell our users how to mount their filesystems, which they're often doing lots of other things with besides running QEMU. (Otherwise we could just tell them "don't use ext4", which would also solve the problem :-)) We need something we can use at the individual-syscall level. thanks -- PMM
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Dec 28, 2018, at 4:18 AM, Peter Maydell wrote: > > On Fri, 28 Dec 2018 at 00:23, Andreas Dilger wrote: >> On Dec 27, 2018, at 10:41 AM, Peter Maydell wrote: >>> As you note, this causes breakage for userspace programs which >>> need to implement an API/ABI with 32-bit offset but which only >>> have access to the kernel's 64-bit offset API/ABI. >> >> This is (IMHO) a bit of an oxymoron, isn't it? Applications using >> the 64-bit API, but storing the value in a 32-bit field? > > I didn't say "which choose to store the value in a 32-bit field", > I said "which have to implement an API/ABI which has 32-bit fields". > In QEMU's case, we use the host kernel's ABI, which has 64-bit > offset fields. We implement a syscall ABI for the guest binary > we are running under emulation, which may have 32-bit offset fields > (for instance if we are running a 32-bit Arm binary.) Both of > these ABIs are fixed -- QEMU doesn't have a choice here, it > just has to make the best effort it can with what the host kernel > provides it, to provide the semantics the guest binary needs. > My suggestion in this thread is that the host kernel provides > a wider range of facilities so that QEMU can do the job it's > trying to do. > >> The same >> problem would exist for filesystems with 64-bit inodes or 64-bit >> file offsets trying to store these values in 32-bit variables. >> It might work most of the time, but it can also break randomly. > > In general inodes and offsets start from 0 and work up -- > so almost all of the time they don't actually overflow. > The problem with ext4 directory hash "offsets" is that they > overflow all the time and immediately, so instead of "works > unless you have a weird edge case" like all the other filesystems, > it's "never works". > >>> I think the best fix for this would be for the kernel to either >>> (a) consistently use a 32-bit hash or (b) to provide an API >>> so that userspace can use the FMODE_32BITHASH flag the way >>> that kernel-internal users already can. >> >> It would be relatively straight forward to add a "32bitapi" mount >> option to return a 32-bit directory hash to userspace for operations >> on that mountpoint (ext4 doesn't have 64-bit inode numbers yet). >> However, I can't think of an easy way to do this on a per-process >> basis without just having it call the 32-bit API directly. > > The problem is that there is no 32-bit API in some cases > (unless I have misunderstood the kernel code) -- not all > host architectures implement compat syscalls or allow them > to be called from 64-bit processes or implement all the older > syscall variants that had smaller offets. If there was a guaranteed > "this syscall always exists and always gives me 32-bit offsets" > we could use it. The "32bitapi" mount option would use 32-bit hash for seekdir and telldir, regardless of what kernel API was used. That would just set the FMODE_32BITHASH flag in the file->f_mode for all files. Using 32-bit directory hash values is not necessarily harmful, but it returns the possibility to hit the problem with hash collisions that previously existed before the move to 64-bit hash values. This becomes more of a problem as directory sizes increase. >> For ext4 at least, you could just shift the high 32-bit part of >> the 64-bit hash down into a 32-bit value in telldir(), and >> shift it back up when seekdir() is called. > > Yes, that has been suggested, but it seemed a bit dubious > to bake in knowledge of ext4's internal implementation details. > Can we rely on this as an ABI promise that will always work > for all versions of all file systems going forwards? Well, the directory cookies need to be relatively stable over time because they are exported to applications and possibly remote nodes via NFS, so it can't be changed very much. Cheers, Andreas signature.asc Description: Message signed with OpenPGP
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Fri, 28 Dec 2018 at 00:23, Andreas Dilger wrote: > On Dec 27, 2018, at 10:41 AM, Peter Maydell wrote: > > As you note, this causes breakage for userspace programs which > > need to implement an API/ABI with 32-bit offset but which only > > have access to the kernel's 64-bit offset API/ABI. > > This is (IMHO) a bit of an oxymoron, isn't it? Applications using > the 64-bit API, but storing the value in a 32-bit field? I didn't say "which choose to store the value in a 32-bit field", I said "which have to implement an API/ABI which has 32-bit fields". In QEMU's case, we use the host kernel's ABI, which has 64-bit offset fields. We implement a syscall ABI for the guest binary we are running under emulation, which may have 32-bit offset fields (for instance if we are running a 32-bit Arm binary.) Both of these ABIs are fixed -- QEMU doesn't have a choice here, it just has to make the best effort it can with what the host kernel provides it, to provide the semantics the guest binary needs. My suggestion in this thread is that the host kernel provides a wider range of facilities so that QEMU can do the job it's trying to do. > The same > problem would exist for filesystems with 64-bit inodes or 64-bit > file offsets trying to store these values in 32-bit variables. > It might work most of the time, but it can also break randomly. In general inodes and offsets start from 0 and work up -- so almost all of the time they don't actually overflow. The problem with ext4 directory hash "offsets" is that they overflow all the time and immediately, so instead of "works unless you have a weird edge case" like all the other filesystems, it's "never works". > > I think the best fix for this would be for the kernel to either > > (a) consistently use a 32-bit hash or (b) to provide an API > > so that userspace can use the FMODE_32BITHASH flag the way > > that kernel-internal users already can. > > It would be relatively straight forward to add a "32bitapi" mount > option to return a 32-bit directory hash to userspace for operations > on that mountpoint (ext4 doesn't have 64-bit inode numbers yet). > However, I can't think of an easy way to do this on a per-process > basis without just having it call the 32-bit API directly. The problem is that there is no 32-bit API in some cases (unless I have misunderstood the kernel code) -- not all host architectures implement compat syscalls or allow them to be called from 64-bit processes or implement all the older syscall variants that had smaller offets. If there was a guaranteed "this syscall always exists and always gives me 32-bit offsets" we could use it. > For ext4 at least, you could just shift the high 32-bit part of > the 64-bit hash down into a 32-bit value in telldir(), and > shift it back up when seekdir() is called. Yes, that has been suggested, but it seemed a bit dubious to bake in knowledge of ext4's internal implementation details. Can we rely on this as an ABI promise that will always work for all versions of all file systems going forwards? thanks -- PMM
Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Dec 27, 2018, at 10:41 AM, Peter Maydell wrote:
>
> On Thu, 27 Dec 2018 at 17:19, Florian Weimer wrote:
>> We have a bit of an interesting problem with respect to the d_off
>> field in struct dirent.
>>
>> When running a 64-bit kernel on certain file systems, notably ext4,
>> this field uses the full 63 bits even for small directories (strace -v
>> output, wrapped here for readability):
>>
>> getdents(3, [
>> {d_ino=1494304, d_off=3901177228673045825, d_reclen=40,
>> d_name="authorized_keys", d_type=DT_REG},
>> {d_ino=1494277, d_off=7491915799041650922, d_reclen=24, d_name=".",
>> d_type=DT_DIR},
>> {d_ino=1314655, d_off=9223372036854775807, d_reclen=24, d_name="..",
>> d_type=DT_DIR}
>> ], 32768) = 88
>>
>> When running in 32-bit compat mode, this value is somehow truncated to
>> 31 bits, for both the getdents and the getdents64 (!) system call (at
>> least on i386).
>
> Yes -- look for hash2pos() and friends in fs/ext4/dir.c.
> The ext4 code in the kernel uses a 32 bit hash if (a) the kernel
> is 32 bit (b) this is a compat syscall (b) some other bit of
> the kernel asked it to via the FMODE_32BITHASH flag (currently only
> NFS does that I think).
>
> As you note, this causes breakage for userspace programs which
> need to implement an API/ABI with 32-bit offset but which only
> have access to the kernel's 64-bit offset API/ABI.
This is (IMHO) a bit of an oxymoron, isn't it? Applications using
the 64-bit API, but storing the value in a 32-bit field? The same
problem would exist for filesystems with 64-bit inodes or 64-bit
file offsets trying to store these values in 32-bit variables.
It might work most of the time, but it can also break randomly.
> I think the best fix for this would be for the kernel to either
> (a) consistently use a 32-bit hash or (b) to provide an API
> so that userspace can use the FMODE_32BITHASH flag the way
> that kernel-internal users already can.
It would be relatively straight forward to add a "32bitapi" mount
option to return a 32-bit directory hash to userspace for operations
on that mountpoint (ext4 doesn't have 64-bit inode numbers yet).
However, I can't think of an easy way to do this on a per-process
basis without just having it call the 32-bit API directly.
> I couldn't think of or find any existing way for userspace
> to get the right results here, which is why
> 32-bit-guest-on-64-bit-host QEMU doesn't work on these filesystems
> (depending on what exactly the guest's libc etc do).
>
>> the 32-bit getdents system call emulation in a 64-bit qemu-user
>> process would just silently truncate the d_off field as part of
>> the translation, not reporting an error.
>> [...]
>> This truncation has always been a bug; it breaks telldir/seekdir
>> at least in some cases.
>
> Yes; you can't fit a quart into a pint pot, so if the guest
> only handles 32-bit offsets then truncation is about all we
> can do. This works fine if offsets are offsets, assuming the
> directory isn't so enormous it would have broken the guest
> anyway. I'm not aware of any issues with this other than the
> oddball ext4 offsets-are-hashes situation -- could you expand
> on the telldir/seekdir issue? (I suppose we should probably
> make QEMU's syscall emulation layer return "no more entries"
> rather than entries with truncated hashes.)
For ext4 at least, you could just shift the high 32-bit part of
the 64-bit hash down into a 32-bit value in telldir(), and
shift it back up when seekdir() is called.
Cheers, Andreas
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Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
On Thu, 27 Dec 2018 at 17:19, Florian Weimer wrote:
> We have a bit of an interesting problem with respect to the d_off
> field in struct dirent.
>
> When running a 64-bit kernel on certain file systems, notably ext4,
> this field uses the full 63 bits even for small directories (strace -v
> output, wrapped here for readability):
>
> getdents(3, [
> {d_ino=1494304, d_off=3901177228673045825, d_reclen=40,
> d_name="authorized_keys", d_type=DT_REG},
> {d_ino=1494277, d_off=7491915799041650922, d_reclen=24, d_name=".",
> d_type=DT_DIR},
> {d_ino=1314655, d_off=9223372036854775807, d_reclen=24, d_name="..",
> d_type=DT_DIR}
> ], 32768) = 88
>
> When running in 32-bit compat mode, this value is somehow truncated to
> 31 bits, for both the getdents and the getdents64 (!) system call (at
> least on i386).
Yes -- look for hash2pos() and friends in fs/ext4/dir.c.
The ext4 code in the kernel uses a 32 bit hash if (a) the kernel
is 32 bit (b) this is a compat syscall (b) some other bit of
the kernel asked it to via the FMODE_32BITHASH flag (currently only
NFS does that I think).
As you note, this causes breakage for userspace programs which
need to implement an API/ABI with 32-bit offset but which only
have access to the kernel's 64-bit offset API/ABI.
I think the best fix for this would be for the kernel to either
(a) consistently use a 32-bit hash or (b) to provide an API
so that userspace can use the FMODE_32BITHASH flag the way
that kernel-internal users already can.
I couldn't think of or find any existing way for userspace
to get the right results here, which is why
32-bit-guest-on-64-bit-host QEMU doesn't work on these filesystems
(depending on what exactly the guest's libc etc do).
> the 32-bit getdents system call emulation in a 64-bit qemu-user
> process would just silently truncate the d_off field as part of
> the translation, not reporting an error.
> [...]
> This truncation has always been a bug; it breaks telldir/seekdir
> at least in some cases.
Yes; you can't fit a quart into a pint pot, so if the guest
only handles 32-bit offsets then truncation is about all we
can do. This works fine if offsets are offsets, assuming the
directory isn't so enormous it would have broken the guest
anyway. I'm not aware of any issues with this other than the
oddball ext4 offsets-are-hashes situation -- could you expand
on the telldir/seekdir issue? (I suppose we should probably
make QEMU's syscall emulation layer return "no more entries"
rather than entries with truncated hashes.)
thanks
-- PMM
