[PATCH] linux: Inline syscall cancellation to keep wrapper frames observable (BZ 34338)

[Adhemerval Zanella Netto]

On 04/08/26 12:26, Andreas Schwab wrote:
> On Jun 30 2026, Adhemerval Zanella wrote:
> 
>> The cancellable syscall wrappers end with a tail call to __syscall_cancel,
>> ithe wrapper frame is then elided, so when the syscall executes the wrapper
>   the

Ack.

> 
>> is no longer present on the stack.  Tools that unwind from CFI alone, such
>> as valgrind, perf and sampling profilers, cannot observe it. On gdb, it
>> only recovers it from DWARF call site information, which reduced-debuginfo
>> libc builds usually omit.
>>
>> The behaviour is target dependent: for a shared (PIC) the tail call is
>> emitted on aarch64, arc, loongarch and riscv.  It is not emitte on i386,
>                                                            emitted

Ack.

> 
>> x86_64, arm, s390x, sparc and alpha, where the seventh argument is passed
>> on the stack or fewer argument registers are available, nor on powerpc
>> and mips, where the TOC/GOT pointer must be restored after the call.
>> This is why the problem was originally reported as aarch64 specific while
>> x86_64 was unaffected.
>>
>> Rather than only inhibiting the tail call [1] (which keeps the wrapper frame
>> but still leaves the __syscall_cancel and __internal_syscall_cancel
>> frames), move the cancellation logic back into the wrappers.  In the
>> single-threaded case the syscall is now issued directly from the wrapper;
>> only the multi-threaded path still calls the out-of-line __syscall_cancel_arch.
>>
>> This keeps the wrapper observable and removes the extra frames, mimicking
>> how cancellation was handled before 89b53077d2a58f00e7debdfe58afabe953dac60d.
>>
>> The result is a small libc.so .text increase (size, first column):
>>
>>   ABI            master      patched      diff    increase
>>   aarch64       1635880     1647424      11544      0.71%
>>   x86_64        1981081     1992257      11176      0.56%
>>   powerpc64le   2364336     2376964      12628      0.53%
>>   riscv64       1368386     1376704       8318      0.61%
>>   loongarch64   1741385     1755601      14216      0.82%
>>
>> The tst-backtrace5 was suppose to track this issue, but due wrong
>> loop variable check it does take this in account.  This patch also fixes it.
> 
> ITYM "does not take".

Ack.

> 
> Ok with that change.
> 

Thanks.