Re: [PATCH v4 01/25] rseq: Introduce feature size and alignment ELF auxiliary vector entries

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2022-10-17 12:09, Mathieu Desnoyers wrote:
> On 2022-10-10 08:42, Florian Weimer wrote:
>> * Mathieu Desnoyers:
>>
>>> Export the rseq feature size supported by the kernel as well as the
>>> required allocation alignment for the rseq per-thread area to user-space
>>> through ELF auxiliary vector entries.
>>>
>>> This is part of the extensible rseq ABI.
>>>
>>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>> ---
>>>   fs/binfmt_elf.c             | 5 +++++
>>>   include/uapi/linux/auxvec.h | 2 ++
>>>   include/uapi/linux/rseq.h   | 5 +++++
>>>   3 files changed, 12 insertions(+)
>>>
>>> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
>>> index 63c7ebb0da89..04fca1e4cbd2 100644
>>> --- a/fs/binfmt_elf.c
>>> +++ b/fs/binfmt_elf.c
>>> @@ -46,6 +46,7 @@
>>>   #include <linux/cred.h>
>>>   #include <linux/dax.h>
>>>   #include <linux/uaccess.h>
>>> +#include <linux/rseq.h>
>>>   #include <asm/param.h>
>>>   #include <asm/page.h>
>>> @@ -288,6 +289,10 @@ create_elf_tables(struct linux_binprm *bprm, 
>>> const struct elfhdr *exec,
>>>       if (bprm->have_execfd) {
>>>           NEW_AUX_ENT(AT_EXECFD, bprm->execfd);
>>>       }
>>> +#ifdef CONFIG_RSEQ
>>> +    NEW_AUX_ENT(AT_RSEQ_FEATURE_SIZE, offsetof(struct rseq, end));
>>> +    NEW_AUX_ENT(AT_RSEQ_ALIGN, __alignof__(struct rseq));
>>> +#endif
>>>   #undef NEW_AUX_ENT
>>>       /* AT_NULL is zero; clear the rest too */
>>>       memset(elf_info, 0, (char *)mm->saved_auxv +
>>> diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
>>> index c7e502bf5a6f..6991c4b8ab18 100644
>>> --- a/include/uapi/linux/auxvec.h
>>> +++ b/include/uapi/linux/auxvec.h
>>> @@ -30,6 +30,8 @@
>>>                    * differ from AT_PLATFORM. */
>>>   #define AT_RANDOM 25    /* address of 16 random bytes */
>>>   #define AT_HWCAP2 26    /* extension of AT_HWCAP */
>>> +#define AT_RSEQ_FEATURE_SIZE    27    /* rseq supported feature size */
>>> +#define AT_RSEQ_ALIGN        28    /* rseq allocation alignment */
>>>   #define AT_EXECFN  31    /* filename of program */
>>
>> Do we need the alignment?  Or can we keep it perpetually at 32?  Or we
>> could steal some bits from AT_RSEQ_FEATURE_SIZE?  (Not the lower
>> bits—they aren't unused due to the way the feature size works.)
> 
> I cannot imagine a use-case that would require us to bump the alignment 
> requirement over 32 bytes, so we may very well leave it at 32. But 
> perhaps someone else has a better imagination than mine ?

Actually, here is a scenario that warrants exposing the required alignment:

Note that struct rseq is *not* packed.

If we extend struct rseq to a size that makes the compiler use an 
alignment larger than 32 bytes in the future, and if the compiler uses 
that larger alignment knowledge to issue instructions that require the 
larger alignment, then it would be incorrect for user-space to allocate 
the struct rseq on an alignment lower than the required alignment.

Indeed, on rseq registration, we have the following check:

if (!IS_ALIGNED((unsigned long)rseq, __alignof__(*rseq))
[...]
    return -EINVAL;

Which would break if the size of struct rseq is large enough that the 
alignment grows larger than 32 bytes.

You mentioned we could steal some high bits from AT_RSEQ_FEATURE_SIZE to 
put the alignment. What is the issue with exposing an explicit 
AT_RSEQ_ALIGN ? It's just a auxv entry, so I don't see it as a huge 
performance concern to access 2 entries rather than one.

Thanks,

Mathieu

> 
> Thanks,
> 
> Mathieu
> 
>>
>> Thanks,
>> Florian
>>
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com