Re: [PATCH v3] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with available page shifts

[Kees Cook]

On Thu, Dec 07, 2023 at 09:44:33PM +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.
> 
> Main users are supposed to be libhugetlbfs-like libraries which try
> to abstract huge mappings across multiple architectures. Regular code
> which queries hugepage support before using them benefits too because
> it doesn't have to deal with descriptors and parsing sysfs hierarchies
> while enjoying the simplicity and speed of getauxval(AT_PAGE_SHIFT_MASK).
> 
> Note!
> 
> This is strictly for userspace, if some page size is shutdown due
> to kernel command line option or CPU bug workaround, than it must
> not be reported in aux vector!
> 
> 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 always reports one smallest page size which is not interesting.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> ---
> 
> 	v3: better comment and changelog
> 	v2: switch to page shifts, rename to ARCH_AT_PAGE_SHIFT_MASK
> 
>  arch/x86/include/asm/elf.h  |   12 ++++++++++++
>  fs/binfmt_elf.c             |    3 +++
>  include/uapi/linux/auxvec.h |   13 +++++++++++++
>  3 files changed, 28 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 */
>  
>  #define VDSO_CURRENT_BASE	((unsigned long)current->mm->context.vdso)

If I can get an Ack from x86 maintainers for this, I can carry it in my
execve tree.

Thanks for the updates to the commit log and comments, it reads better
now.

-Kees

> --- 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
>  	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 +33,19 @@
>  #define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
>  #define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
>  
> +/*
> + * All page sizes supported by CPU encoded as bitmask.
> + *
> + * Example: x86_64 system with pse, pdpe1gb /proc/cpuinfo flags
> + * reports 4 KiB, 2 MiB and 1 GiB page support.
> + *
> + *	$ LD_SHOW_AUXV=1 $(which true) | grep -e AT_PAGE_SHIFT_MASK
> + *	AT_PAGE_SHIFT_MASK: 0x40201000
> + *
> + * For 2^64 hugepage support please contact your Universe sales representative.
> + */
> +#define AT_PAGE_SHIFT_MASK	29
> +
>  #define AT_EXECFN  31	/* filename of program */
>  
>  #ifndef AT_MINSIGSTKSZ

-- 
Kees Cook