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

[Kees Cook <keescook@chromium.org>]

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