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Sun, 17 May 2026 04:45:14 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sun, 17 May 2026 13:45:13 +0200 To: Andrii Nakryiko Cc: Oleg Nesterov , Peter Zijlstra , Ingo Molnar , Masami Hiramatsu , Andrii Nakryiko , bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH 1/7] uprobes/x86: Move optimized uprobe from nop5 to nop10 Message-ID: References: <20260514135342.22130-1-jolsa@kernel.org> <20260514135342.22130-2-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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, May 15, 2026 at 01:31:31PM -0700, Andrii Nakryiko wrote: > On Thu, May 14, 2026 at 6:53 AM Jiri Olsa wrote: > > > > Andrii reported an issue with optimized uprobes [1] that can clobber > > redzone area with call instruction storing return address on stack > > where user code may keep temporary data without adjusting rsp. > > > > Fixing this by moving the optimized uprobes on top of 10-bytes nop > > instruction, so we can squeeze another instruction to escape the > > redzone area before doing the call, like: > > > > lea -0x80(%rsp), %rsp > > call tramp > > > > Note the lea instruction is used to adjust the rsp register without > > changing the flags. > > I think it should be very loudly explained that we can't go back to > nop10 and have to do short jump over patched sequence (and why). there's comment in swbp_unoptimize: * We have optimized nop10 (lea, call), changing it to 'jmp rel8' to * end of the 10-byte slot instead of restoring the original nop10, * because we could have thread already inside lea instruction. I'll add it in here as well > > > > > The optimized uprobe performance stays the same: > > > > uprobe-nop : 3.129 ± 0.013M/s > > uprobe-push : 3.045 ± 0.006M/s > > uprobe-ret : 1.095 ± 0.004M/s > > --> uprobe-nop10 : 7.170 ± 0.020M/s > > uretprobe-nop : 2.143 ± 0.021M/s > > uretprobe-push : 2.090 ± 0.000M/s > > uretprobe-ret : 0.942 ± 0.000M/s > > --> uretprobe-nop10: 3.381 ± 0.003M/s > > usdt-nop : 3.245 ± 0.004M/s > > --> usdt-nop10 : 7.256 ± 0.023M/s > > > > [1] https://lore.kernel.org/bpf/20260509003146.976844-1-andrii@kernel.org/ > > Reported-by: Andrii Nakryiko > > Closes: https://lore.kernel.org/bpf/20260509003146.976844-1-andrii@kernel.org/ > > Fixes: ba2bfc97b462 ("uprobes/x86: Add support to optimize uprobes") > > Signed-off-by: Jiri Olsa > > --- > > arch/x86/kernel/uprobes.c | 121 +++++++++++++++++++++++++++----------- > > 1 file changed, 86 insertions(+), 35 deletions(-) > > > > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c > > index ebb1baf1eb1d..f7c4101a4039 100644 > > --- a/arch/x86/kernel/uprobes.c > > +++ b/arch/x86/kernel/uprobes.c > > @@ -636,9 +636,21 @@ struct uprobe_trampoline { > > unsigned long vaddr; > > }; > > > > +#define LEA_INSN_SIZE 5 > > +#define OPT_INSN_SIZE (LEA_INSN_SIZE + CALL_INSN_SIZE) > > +#define OPT_JMP8_OFFSET (OPT_INSN_SIZE - JMP8_INSN_SIZE) > > +#define REDZONE_SIZE 0x80 > > + > > +static const u8 lea_rsp[] = { 0x48, 0x8d, 0x64, 0x24, 0x80 }; > > + > > +static bool is_lea_insn(const uprobe_opcode_t *insn) > > +{ > > + return !memcmp(insn, lea_rsp, LEA_INSN_SIZE); > > +} > > + > > static bool is_reachable_by_call(unsigned long vtramp, unsigned long vaddr) > > { > > - long delta = (long)(vaddr + 5 - vtramp); > > + long delta = (long)(vaddr + OPT_INSN_SIZE - vtramp); > > > > return delta >= INT_MIN && delta <= INT_MAX; > > } > > @@ -651,7 +663,7 @@ static unsigned long find_nearest_trampoline(unsigned long vaddr) > > }; > > unsigned long low_limit, high_limit; > > unsigned long low_tramp, high_tramp; > > - unsigned long call_end = vaddr + 5; > > + unsigned long call_end = vaddr + OPT_INSN_SIZE; > > > > if (check_add_overflow(call_end, INT_MIN, &low_limit)) > > low_limit = PAGE_SIZE; > > @@ -826,8 +838,8 @@ SYSCALL_DEFINE0(uprobe) > > should we change -ENXIO to -EPROTO or some other distinct error code, > so libbpf can avoid using nop5 attachment on kernels new enough to > support nop5 optimization, but old enough to not do this properly with > nop10? right, I'll take that change as well thanks, jirka