linux-kernel - Re: [RFC RESEND v2] binfmt_elf: preserve original ELF e

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Message-ID: <202508251009.CB5EB2E304@keescook>
Date: Mon, 25 Aug 2025 10:17:07 -0700
From: Kees Cook <kees@...nel.org>
To: Svetlana Parfenova <svetlana.parfenova@...tacore.com>
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 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
> 

-- 
Kees Cook