Re: [PATCHv3 11/12] selftests/bpf: Add tests for uprobe nop10 red zone clobbering

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, May 21, 2026 at 01:26:07PM +0000, bot+bpf-ci@kernel.org wrote:
> > diff --git a/tools/testing/selftests/bpf/prog_tests/uprobe_syscall.c b/tools/testing/selftests/bpf/prog_tests/uprobe_syscall.c
> > index 969f4deba9fd..efff0c515184 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/uprobe_syscall.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/uprobe_syscall.c
> 
> [ ... ]
> 
> > diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
> > index fda3a298ccfc..8004c9568ffa 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/usdt.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c
> 
> [ ... ]
> 
> > diff --git a/tools/testing/selftests/bpf/progs/test_usdt.c b/tools/testing/selftests/bpf/progs/test_usdt.c
> > index f00cb52874e0..0ee78fb050a1 100644
> > --- a/tools/testing/selftests/bpf/progs/test_usdt.c
> > +++ b/tools/testing/selftests/bpf/progs/test_usdt.c
> 
> [ ... ]
> 
> > diff --git a/tools/testing/selftests/bpf/usdt_2.c b/tools/testing/selftests/bpf/usdt_2.c
> > index b359b389f6c0..5e38f8605b02 100644
> > --- a/tools/testing/selftests/bpf/usdt_2.c
> > +++ b/tools/testing/selftests/bpf/usdt_2.c
> > @@ -13,4 +13,17 @@ void usdt_2(void)
> >  	USDT(optimized_attach, usdt_2);
> >  }
> >
> > +static volatile unsigned long usdt_red_zone_arg1 = 0xDEADBEEF;
> > +static volatile unsigned long usdt_red_zone_arg2 = 0xCAFEBABE;
> > +static volatile unsigned long usdt_red_zone_arg3 = 0xFEEDFACE;
> > +
> > +void __attribute__((noinline)) usdt_red_zone_trigger(void)
> > +{
> > +	unsigned long a1 = usdt_red_zone_arg1;
> > +	unsigned long a2 = usdt_red_zone_arg2;
> > +	unsigned long a3 = usdt_red_zone_arg3;
> > +
> > +	USDT(optimized_attach, usdt_red_zone, a1, a2, a3);
> > +}
> 
> Following up on a review comment from sashiko-bot: is this test reliable
> across different compiler versions?

Andrii replied this one in the original post:
  https://lore.kernel.org/bpf/CAEf4BzZ-nASpoF+-vLYZ1D8eWRVd5JWAuhrwx8XZNrX7ZPY5_Q@mail.gmail.com/

jirka

> 
> The x86-64 USDT macro uses the "nor" inline assembly constraint, which allows
> the compiler to allocate these operands to CPU registers. Because there is no
> register pressure in usdt_red_zone_trigger(), modern compilers might allocate
> the local variables a1, a2, and a3 directly to registers rather than the
> stack's red zone.
> 
> If the operands are placed in registers, the uprobe CALL optimization
> clobbering [rsp-8] will not corrupt them. This could cause the test to pass
> even on buggy kernels, creating a false positive.
> 
> The author acknowledged this concern but kept the test as-is, noting that
> selftests compile with -O0 and there's also an assembly-based test.
> 
> Would it be safer to explicitly force these operands into the red zone using
> inline assembly constraints, rather than depending on the compiler's register
> allocator?
> 
> Reference:
> https://lore.kernel.org/bpf/20260509021238.AE98FC2BCB0@smtp.kernel.org/
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/26227520868