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Mon, 18 May 2026 00:30:21 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Mon, 18 May 2026 09:30:19 +0200 To: bot+bpf-ci@kernel.org Cc: oleg@redhat.com, peterz@infradead.org, mingo@kernel.org, mhiramat@kernel.org, andrii@kernel.org, bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org, eddyz87@gmail.com, yonghong.song@linux.dev, clm@meta.com, ihor.solodrai@linux.dev Subject: Re: [PATCH 7/7] selftests/bpf: Add tests for uprobe nop10 red zone clobbering Message-ID: References: <20260514135342.22130-8-jolsa@kernel.org> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, May 14, 2026 at 02:55:27PM +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 82b3c0ce9253..d553485e7db5 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 be34c4087ff5..606601ccdc42 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); > > +} > > + > > #endif > > Is this test reliable across different compiler versions and optimizations? > > The 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 > 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. > > 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? answered by Andrii in: https://lore.kernel.org/bpf/CAEf4BzZ-nASpoF+-vLYZ1D8eWRVd5JWAuhrwx8XZNrX7ZPY5_Q@mail.gmail.com/ jirka