Message-ID: <e9990237-bc83-4cbb-bab8-013b939a61fb@syntacore.com>
Date: Thu, 7 Aug 2025 19:13:50 +0600
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] binfmt_elf: preserve original ELF e_flags in core
 dumps

On 07/08/2025 00.57, Kees Cook wrote:
> 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?
> 

The Kconfig option is currently restricted to the RISC-V architecture 
because it's not clear to me whether other architectures need actual 
e_flags value from ELF header. If this option is disabled, the core dump 
will always use a compile time value for e_flags, regardless of which 
method is selected: ELF_CORE_EFLAGS or CORE_DUMP_USE_REGSET. And this 
constant does not necessarily reflect the actual e_flags of the running 
process (at least on RISC-V), which can vary depending on how the binary 
was compiled. Thus, I made a third method to obtain e_flags that 
reflects the real value. And it is gated behind a Kconfig option, as not 
all users may need it.

>> 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@...tacore.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...
> 

As far as I can tell, the ELF header itself is not preserved beyond 
loading. If there's a mechanism I'm missing that saves it, please let me 
know.

>> +
>>   	/**
>>   	 * 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?
> 

view->e_flags is set at compile time, and view is pointing to const 
struct. The override of e_flags is intentional in both cases 
(ELF_CORE_EFLAGS and CORE_DUMP_USE_REGSET) to allow access to the 
process actual e_flags, regardless of the selected method.

>> +
>> +	/*
>> +	 * 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
> 


-- 
Best regards,
Svetlana Parfenova

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