Re: BUG: rseq selftests and librseq vs. glibc fail

[Prakash Sangappa]

> On Aug 18, 2025, at 10:13 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> On Mon, Aug 18 2025 at 16:15, Florian Weimer wrote:
>> * Thomas Gleixner:
>>> It's trivial to reproduce. All it needs is to have in the source:
>>> 
>>> __weak ptrdiff_t __rseq_offset;
>>> 
>>> w/o even being referenced and creating a pthread. Reproducer below.
>> 
>> Well, that's sort of expected.  You can't define glibc symbols that are
>> not intended for interposition and expect things to work.  It's kind of
>> like writing:
>> 
>> int _rtld_global;
>> 
>> That's going to fail rather spectaculary, too.  We make an exception for
>> symbols that are not reserved (you can build in ISO C mode and define
>> open, close, etc., at least as long as you link to glibc only).  But
>> __rseq_offset is a reserved name, so that is not applicable here.
>> 
>> The real change here is GCC changing from -fcommon (which made a lot of
>> these things work in the past) to -fno-common.
> 
> Thanks for the explanation!
> 
> So the only way to make this actually work is to revert that commit and
> the folks who want to link that statically need to come up with:
> 
> #ifdef _BUILD_STATICALLY
> extern ....
> 
> #else
>        ptr = dlsym(...);
> #endif
> 
> or something daft like that. A proper function interface would avoid all
> that nonsense, but we can't have nice things or can we?


Could the rseq(2) syscall itself return the already registered rseq structure address?
Perhaps a new flag argument to the rseq(2) syscall to query the registered rseq address
or return the address of the already registered rseq structure when it fails to register a new one.

Application can call it when the call to register a rseq structure fails.

-Prakash

> 
> Thanks,
> 
>        tglx
>