Re: [RFC RESEND] binfmt_elf: preserve original ELF e_flags in core dumps

[Kees Cook <kees@kernel.org>]

On Wed, Aug 06, 2025 at 10:18:14PM +0600, Svetlana Parfenova wrote:
> Preserve the original ELF e_flags from the executable in the core dump
> header instead of relying on compile-time defaults (ELF_CORE_EFLAGS or
> value from the regset view). This ensures that ABI-specific flags in
> the dump file match the actual binary being executed.
> 
> 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()). Use this saved value to populate the e_flags in
> the core dump ELF header.
> 
> Add a new Kconfig option, CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS, to guard
> this behavior. Although motivated by a RISC-V use case, the mechanism is
> generic and can be applied to all architectures.

In the general case, is e_flags mismatched? i.e. why hide this behind a
Kconfig? Put another way, if I enabled this Kconfig and dumped core from
some regular x86_64 process, will e_flags be different?

> This change is needed to resolve a debugging issue encountered when
> analyzing core dumps with GDB for RISC-V systems. GDB inspects the
> e_flags field to determine whether optional register sets such as the
> floating-point unit are supported. Without correct flags, GDB may warn
> and ignore valid register data:
> 
>     warning: Unexpected size of section '.reg2/213' in core file.
> 
> As a result, floating-point registers are not accessible in the debugger,
> even though they were dumped. Preserving the original e_flags enables
> GDB and other tools to properly interpret the dump contents.
> 
> Signed-off-by: Svetlana Parfenova <svetlana.parfenova@syntacore.com>
> ---
>  fs/Kconfig.binfmt        |  9 +++++++++
>  fs/binfmt_elf.c          | 26 ++++++++++++++++++++------
>  include/linux/mm_types.h |  5 +++++
>  3 files changed, 34 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
> index bd2f530e5740..45bed2041542 100644
> --- a/fs/Kconfig.binfmt
> +++ b/fs/Kconfig.binfmt
> @@ -184,4 +184,13 @@ config EXEC_KUNIT_TEST
>  	  This builds the exec KUnit tests, which tests boundary conditions
>  	  of various aspects of the exec internals.
>  
> +config CORE_DUMP_USE_PROCESS_EFLAGS
> +	bool "Preserve ELF e_flags from executable in core dumps"
> +	depends on BINFMT_ELF && ELF_CORE && RISCV
> +	default n
> +	help
> +	  Save the ELF e_flags from the process executable at load time
> +	  and use it in the core dump header. This ensures the dump reflects
> +	  the original binary ABI.
> +
>  endmenu
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index caeddccaa1fe..e5e06e11f9fc 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1290,6 +1290,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	mm->end_data = end_data;
>  	mm->start_stack = bprm->p;
>  
> +#ifdef CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS
> +	/* stash e_flags for use in core dumps */
> +	mm->saved_e_flags = elf_ex->e_flags;
> +#endif

Is this structure actually lost during ELF load? I thought we preserved
some more of the ELF headers during load...

> +
>  	/**
>  	 * DOC: "brk" handling
>  	 *
> @@ -1804,6 +1809,8 @@ 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 +1838,24 @@ 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
>  
> +#ifdef CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS
> +	flags = dump_task->mm->saved_e_flags;
> +#endif

This appears to clobber the value from view->e_flags. Is that right? It
feels like this change should only be needed in the default
ELF_CORE_EFLAGS case. How is view->e_flags normally set?

> +
> +	/*
> +	 * 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..39921b32e4f5 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1098,6 +1098,11 @@ struct mm_struct {
>  
>  		unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
>  
> +#ifdef CONFIG_CORE_DUMP_USE_PROCESS_EFLAGS
> +		/* the ABI-related flags from the ELF header. Used for core dump */
> +		unsigned long saved_e_flags;
> +#endif
> +
>  		struct percpu_counter rss_stat[NR_MM_COUNTERS];
>  
>  		struct linux_binfmt *binfmt;
> -- 
> 2.50.1
> 

-Kees

-- 
Kees Cook