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

[Kees Cook]

On August 8, 2025 8:54:30 AM PDT, Svetlana Parfenova <svetlana.parfenova@xxxxxxxxxxxxx> wrote:
>On 08/08/2025 03.14, Kees Cook wrote:
>> On Thu, Aug 07, 2025 at 07:13:50PM +0600, Svetlana Parfenova wrote:
>>> 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.
>> 
>> Can you check if the ELF e_flags and the hard-coded e_flags actually differ on other architectures? I'd rather avoid using the Kconfig so
>> we can have a common execution path for all architectures.
>> 
>
>I checked various architectures, and most don’t use e_flags in core
>dumps - just zero value. For x86 this is valid since it doesn’t define
>values for e_flags. However, architectures like ARM do have meaningful
>e_flags, yet still they are set to zero in core dumps. I guess the real
>question isn't about core dump correctness, but whether tools like GDB
>actually rely on e_flags to provide debug information. Seems like most
>architectures either don’t use it or can operate without it. RISC-V
>looks like black sheep here ... GDB relies on e_flags to determine the
>ABI and interpret the core dump correctly.
>
>What if I rework my patch the following way:
>- remove Kconfig option;
>- add function/macro that would override e_flags with value taken from
>process, but it would only be applied if architecture specifies that.
>
>Would that be a better approach?

Yeah! Let's see what that looks like. :)


-- 
Kees Cook