Re: [PATCHv2 2/2] [fs] proc/vmcore: check the dummy place holder for offline cpu to avoid warning

[Liu ping fan]

On Wed, Dec 21, 2016 at 11:57 AM, Pratyush Anand  wrote:
>
>
> On Wednesday 21 December 2016 08:56 AM, Xunlei Pang wrote:
>>
>> On 12/20/2016 at 11:38 PM, Pratyush Anand wrote:
>>>
>>>
>>>
>>> On Monday 19 December 2016 08:10 AM, Dave Young wrote:

 Hi, Pingfan

 On 12/19/16 at 10:08am, Pingfan Liu wrote:
>>
>> kexec-tools always allocates program headers for present cpus. But
>> when crashing, offline cpus have dummy headers. We do not copy these
>> dummy notes into ELF file, also have no need of warning on them.

 I still think it is not worth such a fix, if you feel a lot of warnings
 in case large cpu numbers, I think you can change the pr_warn to
 pr_warn_once, we do not care the null cpu notes if it has nothing bad
 to the vmcore.

>>>
>>> I agree. Warning is more like information here. May be, we can count the
>>> number of times real_sz was 0, and then can print an info at the end in
>>> stead of warning, like..."N number of CPUs would have been offline, PT_NOTE
>>> entries was absent for them."
>>
>>
>> Well, OTOH the warning may also be due to some user-space misuse, we can't
>> distinguish that without extra information added.
>
>
> Yes, yes..I agree, I meant that the above info is just indicative. May be
> "might have been" could be better word than "would have been" in the above
> info print message.
>
>
>>
>> Another possible user-space fix would be: Firstly fix kexec-tools to add
>> notes only for online cpus,
>> then utilize udev rules(cpu online/offline events) to automatically
>> trigger kdump kernel reload.
>
>
> Hummm..this is certainly possible. But can we do much even when we get the
> info that the PT_NOTE was compromised by user space?
>
If doing this, kexec should be re-forked each time when
online/offline. I think it is not worth to do so just for suppressing
some warning.

> Therefore, I am of the view that if at all we are concerned about number of
> warning messages in case of multiple offline cpu, then we can just print the
> total number of NULL PT_NOTE at the end of loop.
>
Yes, I think it is a good idea. NULL PT_NOTES are only caused by
offlne cpu, it is better to change the waring to info

Thx,
Pingfan

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Re: [PATCHv2 2/2] [fs] proc/vmcore: check the dummy place holder for offline cpu to avoid warning

[Xunlei Pang]

On 12/21/2016 at 11:57 AM, Pratyush Anand wrote:
>
>
> On Wednesday 21 December 2016 08:56 AM, Xunlei Pang wrote:
>> On 12/20/2016 at 11:38 PM, Pratyush Anand wrote:
>>>
>>>
>>> On Monday 19 December 2016 08:10 AM, Dave Young wrote:
 Hi, Pingfan

 On 12/19/16 at 10:08am, Pingfan Liu wrote:
>> kexec-tools always allocates program headers for present cpus. But
>> when crashing, offline cpus have dummy headers. We do not copy these
>> dummy notes into ELF file, also have no need of warning on them.
 I still think it is not worth such a fix, if you feel a lot of warnings
 in case large cpu numbers, I think you can change the pr_warn to
 pr_warn_once, we do not care the null cpu notes if it has nothing bad
 to the vmcore.

>>>
>>> I agree. Warning is more like information here. May be, we can count the 
>>> number of times real_sz was 0, and then can print an info at the end in 
>>> stead of warning, like..."N number of CPUs would have been offline, PT_NOTE 
>>> entries was absent for them."
>>
>> Well, OTOH the warning may also be due to some user-space misuse, we can't 
>> distinguish that without extra information added.
>
> Yes, yes..I agree, I meant that the above info is just indicative. May be 
> "might have been" could be better word than "would have been" in the above 
> info print message.
>
>
>>
>> Another possible user-space fix would be: Firstly fix kexec-tools to add 
>> notes only for online cpus,
>> then utilize udev rules(cpu online/offline events) to automatically trigger 
>> kdump kernel reload.
>
> Hummm..this is certainly possible. But can we do much even when we get the 
> info that the PT_NOTE was compromised by user space?
>
> Therefore, I am of the view that if at all we are concerned about number of 
> warning messages in case of multiple offline cpu, then we can just print the 
> total number of NULL PT_NOTE at the end of loop.

Yes, agree.

Regards,
Xunlei

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Re: [PATCHv2 2/2] [fs] proc/vmcore: check the dummy place holder for offline cpu to avoid warning

[Xunlei Pang]

On 12/20/2016 at 11:38 PM, Pratyush Anand wrote:
>
>
> On Monday 19 December 2016 08:10 AM, Dave Young wrote:
>> Hi, Pingfan
>>
>> On 12/19/16 at 10:08am, Pingfan Liu wrote:
>>> > kexec-tools always allocates program headers for present cpus. But
>>> > when crashing, offline cpus have dummy headers. We do not copy these
>>> > dummy notes into ELF file, also have no need of warning on them.
>> I still think it is not worth such a fix, if you feel a lot of warnings
>> in case large cpu numbers, I think you can change the pr_warn to
>> pr_warn_once, we do not care the null cpu notes if it has nothing bad
>> to the vmcore.
>>
>
> I agree. Warning is more like information here. May be, we can count the 
> number of times real_sz was 0, and then can print an info at the end in stead 
> of warning, like..."N number of CPUs would have been offline, PT_NOTE entries 
> was absent for them."

Well, OTOH the warning may also be due to some user-space misuse, we can't 
distinguish that without extra information added.

Another possible user-space fix would be: Firstly fix kexec-tools to add notes 
only for online cpus,
then utilize udev rules(cpu online/offline events) to automatically trigger 
kdump kernel reload.

Regards,
Xunlei

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Re: [PATCHv2 2/2] [fs] proc/vmcore: check the dummy place holder for offline cpu to avoid warning

[Pratyush Anand]

On Monday 19 December 2016 08:10 AM, Dave Young wrote:

Hi, Pingfan

On 12/19/16 at 10:08am, Pingfan Liu wrote:

> kexec-tools always allocates program headers for present cpus. But
> when crashing, offline cpus have dummy headers. We do not copy these
> dummy notes into ELF file, also have no need of warning on them.

I still think it is not worth such a fix, if you feel a lot of warnings
in case large cpu numbers, I think you can change the pr_warn to
pr_warn_once, we do not care the null cpu notes if it has nothing bad
to the vmcore.



I agree. Warning is more like information here. May be, we can count the 
number of times real_sz was 0, and then can print an info at the end in 
stead of warning, like..."N number of CPUs would have been offline, 
PT_NOTE entries was absent for them."


~Pratyush

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Re: [PATCHv2 2/2] [fs] proc/vmcore: check the dummy place holder for offline cpu to avoid warning

[Dave Young]

Hi, Pingfan

On 12/19/16 at 10:08am, Pingfan Liu wrote:
> kexec-tools always allocates program headers for present cpus. But
> when crashing, offline cpus have dummy headers. We do not copy these
> dummy notes into ELF file, also have no need of warning on them.

I still think it is not worth such a fix, if you feel a lot of warnings
in case large cpu numbers, I think you can change the pr_warn to
pr_warn_once, we do not care the null cpu notes if it has nothing bad
to the vmcore.

> 
> Signed-off-by: Pingfan Liu 
> ---
>  fs/proc/vmcore.c | 46 ++
>  1 file changed, 38 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 8ab782d..fc6e352 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -526,9 +526,10 @@ static u64 __init get_vmcore_size(size_t elfsz, size_t 
> elfnotesegsz,
>   */
>  static int __init update_note_header_size_elf64(const Elf64_Ehdr *ehdr_ptr)
>  {
> - int i, rc=0;
> + int i, j, rc = 0;
>   Elf64_Phdr *phdr_ptr;
> - Elf64_Nhdr *nhdr_ptr;
> + Elf64_Nhdr *nhdr_ptr, *prev_ptr;
> + bool warn;
>  
>   phdr_ptr = (Elf64_Phdr *)(ehdr_ptr + 1);
>   for (i = 0; i < ehdr_ptr->e_phnum; i++, phdr_ptr++) {
> @@ -536,6 +537,7 @@ static int __init update_note_header_size_elf64(const 
> Elf64_Ehdr *ehdr_ptr)
>   u64 offset, max_sz, sz, real_sz = 0;
>   if (phdr_ptr->p_type != PT_NOTE)
>   continue;
> + warn = true;
>   max_sz = phdr_ptr->p_memsz;
>   offset = phdr_ptr->p_offset;
>   notes_section = kmalloc(max_sz, GFP_KERNEL);
> @@ -556,14 +558,27 @@ static int __init update_note_header_size_elf64(const 
> Elf64_Ehdr *ehdr_ptr)
>   nhdr_ptr->n_namesz, nhdr_ptr->n_descsz);
>   break;
>   }
> + prev_ptr = nhdr_ptr;
>   real_sz += sz;
>   nhdr_ptr = (Elf64_Nhdr*)((char*)nhdr_ptr + sz);
> + if ((prev_ptr->n_type == NT_DUMMY)
> +   && !strncmp(KEXEC_CORE_NOTE_NAME,
> + (char *)prev_ptr + sizeof(Elf64_Nhdr),
> + strlen(KEXEC_CORE_NOTE_NAME))) {
> + if (nhdr_ptr->n_namesz == 0) {
> + /* do not copy this dummy note */
> + real_sz = 0;
> + warn = false;
> + } else
> + pr_warn("Warning: Dummy PT_NOTE not 
> overwritten\n");
> + }
>   }
> + if (real_sz != 0)
> + warn = false;
>   kfree(notes_section);
>   phdr_ptr->p_memsz = real_sz;
> - if (real_sz == 0) {
> + if (warn)
>   pr_warn("Warning: Zero PT_NOTE entries found\n");
> - }
>   }
>  
>   return 0;
> @@ -712,9 +727,10 @@ static int __init merge_note_headers_elf64(char *elfptr, 
> size_t *elfsz,
>   */
>  static int __init update_note_header_size_elf32(const Elf32_Ehdr *ehdr_ptr)
>  {
> - int i, rc=0;
> + int i, j, rc = 0;
>   Elf32_Phdr *phdr_ptr;
> - Elf32_Nhdr *nhdr_ptr;
> + Elf32_Nhdr *nhdr_ptr, *prev_ptr;
> + bool warn;
>  
>   phdr_ptr = (Elf32_Phdr *)(ehdr_ptr + 1);
>   for (i = 0; i < ehdr_ptr->e_phnum; i++, phdr_ptr++) {
> @@ -722,6 +738,7 @@ static int __init update_note_header_size_elf32(const 
> Elf32_Ehdr *ehdr_ptr)
>   u64 offset, max_sz, sz, real_sz = 0;
>   if (phdr_ptr->p_type != PT_NOTE)
>   continue;
> + warn = true;
>   max_sz = phdr_ptr->p_memsz;
>   offset = phdr_ptr->p_offset;
>   notes_section = kmalloc(max_sz, GFP_KERNEL);
> @@ -742,14 +759,27 @@ static int __init update_note_header_size_elf32(const 
> Elf32_Ehdr *ehdr_ptr)
>   nhdr_ptr->n_namesz, nhdr_ptr->n_descsz);
>   break;
>   }
> + prev_ptr = nhdr_ptr;
>   real_sz += sz;
>   nhdr_ptr = (Elf32_Nhdr*)((char*)nhdr_ptr + sz);
> + if ((prev_ptr->n_type == NT_DUMMY)
> +   && !strncmp(KEXEC_CORE_NOTE_NAME,
> + (char *)prev_ptr + sizeof(Elf32_Nhdr),
> + strlen(KEXEC_CORE_NOTE_NAME))) {
> + if (nhdr_ptr->n_namesz == 0) {
> + /* do not copy this dummy note */
> + real_sz = 0;
> + warn = false;
> + } else
> + 

[PATCHv2 2/2] [fs] proc/vmcore: check the dummy place holder for offline cpu to avoid warning

[Pingfan Liu]

kexec-tools always allocates program headers for present cpus. But
when crashing, offline cpus have dummy headers. We do not copy these
dummy notes into ELF file, also have no need of warning on them.

Signed-off-by: Pingfan Liu 
---
 fs/proc/vmcore.c | 46 ++
 1 file changed, 38 insertions(+), 8 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 8ab782d..fc6e352 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -526,9 +526,10 @@ static u64 __init get_vmcore_size(size_t elfsz, size_t 
elfnotesegsz,
  */
 static int __init update_note_header_size_elf64(const Elf64_Ehdr *ehdr_ptr)
 {
-   int i, rc=0;
+   int i, j, rc = 0;
Elf64_Phdr *phdr_ptr;
-   Elf64_Nhdr *nhdr_ptr;
+   Elf64_Nhdr *nhdr_ptr, *prev_ptr;
+   bool warn;
 
phdr_ptr = (Elf64_Phdr *)(ehdr_ptr + 1);
for (i = 0; i < ehdr_ptr->e_phnum; i++, phdr_ptr++) {
@@ -536,6 +537,7 @@ static int __init update_note_header_size_elf64(const 
Elf64_Ehdr *ehdr_ptr)
u64 offset, max_sz, sz, real_sz = 0;
if (phdr_ptr->p_type != PT_NOTE)
continue;
+   warn = true;
max_sz = phdr_ptr->p_memsz;
offset = phdr_ptr->p_offset;
notes_section = kmalloc(max_sz, GFP_KERNEL);
@@ -556,14 +558,27 @@ static int __init update_note_header_size_elf64(const 
Elf64_Ehdr *ehdr_ptr)
nhdr_ptr->n_namesz, nhdr_ptr->n_descsz);
break;
}
+   prev_ptr = nhdr_ptr;
real_sz += sz;
nhdr_ptr = (Elf64_Nhdr*)((char*)nhdr_ptr + sz);
+   if ((prev_ptr->n_type == NT_DUMMY)
+ && !strncmp(KEXEC_CORE_NOTE_NAME,
+   (char *)prev_ptr + sizeof(Elf64_Nhdr),
+   strlen(KEXEC_CORE_NOTE_NAME))) {
+   if (nhdr_ptr->n_namesz == 0) {
+   /* do not copy this dummy note */
+   real_sz = 0;
+   warn = false;
+   } else
+   pr_warn("Warning: Dummy PT_NOTE not 
overwritten\n");
+   }
}
+   if (real_sz != 0)
+   warn = false;
kfree(notes_section);
phdr_ptr->p_memsz = real_sz;
-   if (real_sz == 0) {
+   if (warn)
pr_warn("Warning: Zero PT_NOTE entries found\n");
-   }
}
 
return 0;
@@ -712,9 +727,10 @@ static int __init merge_note_headers_elf64(char *elfptr, 
size_t *elfsz,
  */
 static int __init update_note_header_size_elf32(const Elf32_Ehdr *ehdr_ptr)
 {
-   int i, rc=0;
+   int i, j, rc = 0;
Elf32_Phdr *phdr_ptr;
-   Elf32_Nhdr *nhdr_ptr;
+   Elf32_Nhdr *nhdr_ptr, *prev_ptr;
+   bool warn;
 
phdr_ptr = (Elf32_Phdr *)(ehdr_ptr + 1);
for (i = 0; i < ehdr_ptr->e_phnum; i++, phdr_ptr++) {
@@ -722,6 +738,7 @@ static int __init update_note_header_size_elf32(const 
Elf32_Ehdr *ehdr_ptr)
u64 offset, max_sz, sz, real_sz = 0;
if (phdr_ptr->p_type != PT_NOTE)
continue;
+   warn = true;
max_sz = phdr_ptr->p_memsz;
offset = phdr_ptr->p_offset;
notes_section = kmalloc(max_sz, GFP_KERNEL);
@@ -742,14 +759,27 @@ static int __init update_note_header_size_elf32(const 
Elf32_Ehdr *ehdr_ptr)
nhdr_ptr->n_namesz, nhdr_ptr->n_descsz);
break;
}
+   prev_ptr = nhdr_ptr;
real_sz += sz;
nhdr_ptr = (Elf32_Nhdr*)((char*)nhdr_ptr + sz);
+   if ((prev_ptr->n_type == NT_DUMMY)
+ && !strncmp(KEXEC_CORE_NOTE_NAME,
+   (char *)prev_ptr + sizeof(Elf32_Nhdr),
+   strlen(KEXEC_CORE_NOTE_NAME))) {
+   if (nhdr_ptr->n_namesz == 0) {
+   /* do not copy this dummy note */
+   real_sz = 0;
+   warn = false;
+   } else
+   pr_warn("Warning: Dummy PT_NOTE not 
overwritten\n");
+   }
}
+   if (real_sz != 0)
+   warn = false;
kfree(notes_section);
phdr_ptr->p_memsz = real_sz;
-   if (real_sz == 0) {
+   if (warn)
pr_warn("Warning: Zero