Re: [PATCH v2] ELF: supply userspace with available page shifts (AT_PAGE_SHIFT_MASK)

Kees Cook
2023-12-06T20:47:29Z

On Tue, Dec 05, 2023 at 07:01:34PM +0300, Alexey Dobriyan wrote:
> Report available page shifts in arch independent manner, so that
> userspace developers won't have to parse /proc/cpuinfo hunting
> for arch specific strings:
>
> Note!
>
> This is strictly for userspace, if some page size is shutdown due
> to kernel command line option or CPU bug workaround, than is must not
> be reported in aux vector!

Given Florian in CC, I assume this is something glibc would like to be
using? Please mention this in the commit log.

>
> x86_64 machine with 1 GiB pages:
>
> 00000030 06 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00
> 00000040 1d 00 00 00 00 00 00 00 00 10 20 40 00 00 00 00
>
> x86_64 machine with 2 MiB pages only:
>
> 00000030 06 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00
> 00000040 1d 00 00 00 00 00 00 00 00 10 20 00 00 00 00 00
>
> AT_PAGESZ is always 4096 which is not that interesting.

That's not always true. For example, see arm64:
arch/arm64/include/asm/elf.h:#define ELF_EXEC_PAGESIZE PAGE_SIZE

I'm not actually sure why x86 forces it to 4096. I'd need to go look
through the history there.

>
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---
>
> v2: switch to 1 bit per page shift (bitmask)
>
> arch/x86/include/asm/elf.h | 12 ++++++++++++
> fs/binfmt_elf.c | 3 +++
> include/uapi/linux/auxvec.h | 14 ++++++++++++++
> 3 files changed, 29 insertions(+)
>
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -358,6 +358,18 @@ else if (IS_ENABLED(CONFIG_IA32_EMULATION)) \
>
> #define COMPAT_ELF_ET_DYN_BASE (TASK_UNMAPPED_BASE + 0x1000000)
>
> +#define ARCH_AT_PAGE_SHIFT_MASK \
> + do { \
> + u32 val = 1 << 12; \
> + if (boot_cpu_has(X86_FEATURE_PSE)) { \
> + val |= 1 << 21; \
> + } \
> + if (boot_cpu_has(X86_FEATURE_GBPAGES)) { \
> + val |= 1 << 30; \
> + } \
> + NEW_AUX_ENT(AT_PAGE_SHIFT_MASK, val); \
> + } while (0)
> +
> #endif /* !CONFIG_X86_32 */

Can't we have a generic ARCH_AT_PAGE_SHIFT_MASK too? Something like:

#ifndef ARCH_AT_PAGE_SHIFT_MASK
#define ARCH_AT_PAGE_SHIFT_MASK
NEW_AUX_ENT(AT_PAGE_SHIFT_MASK, 1 << PAGE_SHIFT)
#endif

Or am I misunderstanding something here?

>
> #define VDSO_CURRENT_BASE ((unsigned long)current->mm->context.vdso)
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -240,6 +240,9 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
> #endif
> NEW_AUX_ENT(AT_HWCAP, ELF_HWCAP);
> NEW_AUX_ENT(AT_PAGESZ, ELF_EXEC_PAGESIZE);
> +#ifdef ARCH_AT_PAGE_SHIFT_MASK
> + ARCH_AT_PAGE_SHIFT_MASK;
> +#endif

That way we can avoid an #ifdef in the .c file.

> NEW_AUX_ENT(AT_CLKTCK, CLOCKS_PER_SEC);
> NEW_AUX_ENT(AT_PHDR, phdr_addr);
> NEW_AUX_ENT(AT_PHENT, sizeof(struct elf_phdr));
> --- a/include/uapi/linux/auxvec.h
> +++ b/include/uapi/linux/auxvec.h
> @@ -33,6 +33,20 @@
> #define AT_RSEQ_FEATURE_SIZE 27 /* rseq supported feature size */
> #define AT_RSEQ_ALIGN 28 /* rseq allocation alignment */
>
> +/*
> + * Page sizes available for mmap(2) encoded as bitmask.
> + *
> + * Example: x86_64 system with pse, pdpe1gb /proc/cpuinfo flags reports
> + * 4 KiB, 2 MiB and 1 GiB page support.
> + *
> + * $ hexdump -C /proc/self/auxv

FWIW, a more readable form is: $ LD_SHOW_AUXV=1 /bin/true

> + * 00000030 06 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00
> + * 00000040 1d 00 00 00 00 00 00 00 00 10 20 40 00 00 00 00
> + *
> + * For 2^64 hugepage support please contact your Universe sales representative.
> + */
> +#define AT_PAGE_SHIFT_MASK 29

... hmm, why is 29 unused?

> +
> #define AT_EXECFN 31 /* filename of program */
>
> #ifndef AT_MINSIGSTKSZ

This will need a man page update for "getauxval" as well...

--
Kees Cook