linux-kernel - Re: BUG: rseq selftests and librseq vs. glibc fail

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Message-ID: <87cy8sy2n6.ffs@tglx>
Date: Mon, 18 Aug 2025 19:13:01 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Florian Weimer <fweimer@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>,
 Michael Jeanson <mjeanson@...icios.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng
 <boqun.feng@...il.com>, Wei Liu <wei.liu@...nel.org>, Sean
 Christopherson <seanjc@...gle.com>, Samuel Thibault <sthibault@...ian.org>
Subject: Re: BUG: rseq selftests and librseq vs. glibc fail

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?

Thanks,

        tglx