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

Thanks,

Mathieu

>
> Thanks,
> Florian
>

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

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