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Message-ID: <308785c9-5579-4950-aea5-f19d02a2de37@syntacore.com>
Date: Mon, 1 Sep 2025 20:58:56 +0700
From: Svetlana Parfenova <svetlana.parfenova@...tacore.com>
To: Kees Cook <kees@...nel.org>
CC: <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <viro@...iv.linux.org.uk>,
<brauner@...nel.org>, <jack@...e.cz>, <akpm@...ux-foundation.org>,
<david@...hat.com>, <lorenzo.stoakes@...cle.com>, <Liam.Howlett@...cle.com>,
<vbabka@...e.cz>, <rppt@...nel.org>, <surenb@...gle.com>, <mhocko@...e.com>
Subject: Re: [RFC RESEND v2] binfmt_elf: preserve original ELF e_flags for
core dumps
On 26/08/2025 00.17, Kees Cook wrote:
> On Mon, Aug 11, 2025 at 03:53:28PM +0600, Svetlana Parfenova wrote:
>> Some architectures, such as RISC-V, use the ELF e_flags field to encode
>> ABI-specific information (e.g., ISA extensions, fpu support). Debuggers
>> like GDB rely on these flags in core dumps to correctly interpret
>> optional register sets. If the flags are missing or incorrect, GDB may
>> warn and ignore valid data, for example:
>>
>> warning: Unexpected size of section '.reg2/213' in core file.
>>
>> This can prevent access to fpu or other architecture-specific registers
>> even when they were dumped.
>>
>> Save the e_flags field during ELF binary loading (in load_elf_binary())
>> into the mm_struct, and later retrieve it during core dump generation
>> (in fill_note_info()). A new macro ELF_CORE_USE_PROCESS_EFLAGS allows
>> architectures to enable this behavior - currently just RISC-V.
>>
>> Signed-off-by: Svetlana Parfenova <svetlana.parfenova@...tacore.com>
>> ---
>> Changes in v2:
>> - Remove usage of Kconfig option.
>> - Add an architecture-optional macro to set process e_flags. Enabled
>> by defining ELF_CORE_USE_PROCESS_EFLAGS. Defaults to no-op if not
>> used.
>>
>> arch/riscv/include/asm/elf.h | 1 +
>> fs/binfmt_elf.c | 34 ++++++++++++++++++++++++++++------
>> include/linux/mm_types.h | 3 +++
>> 3 files changed, 32 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/elf.h b/arch/riscv/include/asm/elf.h
>> index c7aea7886d22..5d9f0ac851ee 100644
>> --- a/arch/riscv/include/asm/elf.h
>> +++ b/arch/riscv/include/asm/elf.h
>> @@ -20,6 +20,7 @@
>> * These are used to set parameters in the core dumps.
>> */
>> #define ELF_ARCH EM_RISCV
>> +#define ELF_CORE_USE_PROCESS_EFLAGS
>
> Let's move this to the per-arch Kconfig instead, that way we can use it
> in other places. Maybe call in CONFIG_ARCH_HAS_ELF_CORE_EFLAGS?
>
>>
>> #ifndef ELF_CLASS
>> #ifdef CONFIG_64BIT
>> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
>> index caeddccaa1fe..e52b1e077218 100644
>> --- a/fs/binfmt_elf.c
>> +++ b/fs/binfmt_elf.c
>> @@ -66,6 +66,14 @@
>> #define elf_check_fdpic(ex) false
>> #endif
>>
>> +#ifdef ELF_CORE_USE_PROCESS_EFLAGS
>> +#define elf_coredump_get_process_eflags(dump_task, e_flags) \
>> + (*(e_flags) = (dump_task)->mm->saved_e_flags)
>> +#else
>> +#define elf_coredump_get_process_eflags(dump_task, e_flags) \
>> + do { (void)(dump_task); (void)(e_flags); } while (0)
>> +#endif
>
> Let's make specific set/get helpers here, instead.
>
> static inline
> u32 coredump_get_mm_eflags(struct mm_struct *mm, u32 flags)
> {
> #ifdef CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
> flags = mm->saved_e_flags;
> #else
> return flags;
> }
>
> static inline
> void coredump_set_mm_eflags(struct mm_struct *mm, u32 flags)
> {
> #ifdef CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
> mm->saved_e_flags = flags;
> #endif
> }
>
>
>> +
>> static int load_elf_binary(struct linux_binprm *bprm);
>>
>> /*
>> @@ -1290,6 +1298,9 @@ static int load_elf_binary(struct linux_binprm *bprm)
>> mm->end_data = end_data;
>> mm->start_stack = bprm->p;
>>
>> + /* stash e_flags for use in core dumps */
>> + mm->saved_e_flags = elf_ex->e_flags;
>
> Then this is:
>
> coredump_set_mm_eflags(mm, elf_ex->e_flags);
>
>> +
>> /**
>> * DOC: "brk" handling
>> *
>> @@ -1804,6 +1815,8 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
>> struct elf_thread_core_info *t;
>> struct elf_prpsinfo *psinfo;
>> struct core_thread *ct;
>> + u16 machine;
>> + u32 flags;
>>
>> psinfo = kmalloc(sizeof(*psinfo), GFP_KERNEL);
>> if (!psinfo)
>> @@ -1831,17 +1844,26 @@ static int fill_note_info(struct elfhdr *elf, int phdrs,
>> return 0;
>> }
>>
>> - /*
>> - * Initialize the ELF file header.
>> - */
>> - fill_elf_header(elf, phdrs,
>> - view->e_machine, view->e_flags);
>> + machine = view->e_machine;
>> + flags = view->e_flags;
>> #else
>> view = NULL;
>> info->thread_notes = 2;
>> - fill_elf_header(elf, phdrs, ELF_ARCH, ELF_CORE_EFLAGS);
>> + machine = ELF_ARCH;
>> + flags = ELF_CORE_EFLAGS;
>> #endif
>>
>> + /*
>> + * Override ELF e_flags with value taken from process,
>> + * if arch wants to.
>> + */
>> + elf_coredump_get_process_eflags(dump_task, &flags);
>
> And this is:
>
> flags = coredump_get_mm_eflags(dump_task->mm, flags);
>
>> +
>> + /*
>> + * Initialize the ELF file header.
>> + */
>> + fill_elf_header(elf, phdrs, machine, flags);
>> +
>> /*
>> * Allocate a structure for each thread.
>> */
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index d6b91e8a66d6..e46f554f8d91 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -1098,6 +1098,9 @@ struct mm_struct {
>>
>> unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
>>
>
> And then add:
>
> #ifdef CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
>
>> + /* the ABI-related flags from the ELF header. Used for core dump */
>> + unsigned long saved_e_flags;
>
> #endif
>
> around this part
>
>> +
>> struct percpu_counter rss_stat[NR_MM_COUNTERS];
>>
>> struct linux_binfmt *binfmt;
>> --
>> 2.50.1
>>
>
Thank you for review! I have addressed your comments in v3 of the patch.
--
Best regards,
Svetlana Parfenova
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