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[198.0.35.241]) by smtp.gmail.com with ESMTPSA id d6-20020a170903230600b001d347a98e7asm114457plh.260.2023.12.12.13.09.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 Dec 2023 13:09:22 -0800 (PST) Date: Tue, 12 Dec 2023 13:09:21 -0800 From: Kees Cook To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen Cc: Alexey Dobriyan , Andrew Morton , Florian Weimer , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, x86@kernel.org, Eric Biederman , linux-mm@kvack.org Subject: Re: [PATCH v3] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with available page shifts Message-ID: <202312121307.D6605DCD@keescook> References: <6b399b86-a478-48b0-92a1-25240a8ede54@p183> <87v89dvuxg.fsf@oldenburg.str.redhat.com> <1d679805-8a82-44a4-ba14-49d4f28ff597@p183> <8582f7c9-b49d-4d21-8948-59d580e5317c@p183> Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8582f7c9-b49d-4d21-8948-59d580e5317c@p183> 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 > --- > > 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