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Wed, 10 Jun 2026 01:18:52 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Wed, 10 Jun 2026 10:18:51 +0200 To: Andrii Nakryiko Cc: Jiri Olsa , Oleg Nesterov , Peter Zijlstra , Ingo Molnar , Masami Hiramatsu , Andrii Nakryiko , bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCHv4 05/13] uprobes/x86: Move optimized uprobe from nop5 to nop10 Message-ID: References: <20260526205840.173790-1-jolsa@kernel.org> <20260526205840.173790-6-jolsa@kernel.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@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 Tue, Jun 09, 2026 at 09:43:15AM -0700, Andrii Nakryiko wrote: > On Tue, Jun 9, 2026 at 4:44 AM Jiri Olsa wrote: > > > > On Mon, Jun 08, 2026 at 01:46:39PM -0700, Andrii Nakryiko wrote: > > > On Tue, May 26, 2026 at 1:59 PM 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. > > > > > > > > We use nop10 and following transformation to optimized instructions > > > > above and back as suggested by Peterz [2]. > > > > > > > > Optimize path (int3_update_optimize): > > > > > > > > 1) Initial state after set_swbp() installed the uprobe: > > > > cc 2e 0f 1f 84 00 00 00 00 00 > > > > > > > > From offset 0 this is INT3 followed by the tail of the original > > > > 10-byte NOP. > > > > > > > > After a previous unoptimization bytes 5..9 may still contain the > > > > old call instruction, which remains valid for threads already there. > > > > > > > > 2) Rewrite the LEA tail and call displacement: > > > > cc [8d 64 24 80 e8 d0 d1 d2 d3] > > > > > > > > From offset 0 this traps on the uprobe INT3. Bytes 1..9 are not > > > > executable entry points while byte 0 is trapped. > > > > > > > > 3) Publish the first LEA byte: > > > > [48] 8d 64 24 80 e8 d0 d1 d2 d3 > > > > > > > > From offset 0 this is: > > > > lea -0x80(%rsp), %rsp > > > > call > > > > > > > > Unoptimize path (int3_update_unoptimize): > > > > > > > > 1) Initial optimized state: > > > > 48 8d 64 24 80 e8 d0 d1 d2 d3 > > > > Same as 3) above. > > > > > > > > 2) Trap new entries before restoring the NOP bytes: > > > > [cc] 8d 64 24 80 e8 d0 d1 d2 d3 > > > > > > > > From offset 0 this traps. A thread that had already executed the > > > > LEA can still reach the intact CALL at offset 5. > > > > > > > > 3) Restore bytes 1..4 of the original NOP while keeping byte 0 trapped > > > > and byte 5 as CALL. > > > > cc [2e 0f 1f 84] e8 d0 d1 d2 d3 > > > > > > > > From offset 0 this still traps. Offset 5 is still the CALL for any > > > > thread that was already past the first LEA byte. > > > > > > > > 4) Publish the first byte of the original NOP: > > > > [66] 2e 0f 1f 84 e8 d0 d1 d2 d3 > > > > > > > > From offset 0 this is the restored 10-byte NOP; the CALL opcode and > > > > displacement are now only NOP operands. Offset 5 still decodes as > > > > CALL for a thread that was already there. > > > > > > > > Tthere is only a single target uprobe-trampoline for the given nop10 > > > > instruction address, so the CALL instruction will not be changed across > > > > unoptimization/optimization cycles. > > > > Therefore, any task that is preempted at the CALL instruction is guaranteed > > > > to observe that CALL and not anything else. > > > > > > > > Note as explained in [2] we need to use following nop10: > > > > PF1 PF2 ESC NOPL MOD SIB DISP32 > > > > NOP10: 0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00 -- cs nopw 0x00000000(%rax,%rax,1) > > > > > > > > which means we need to allow 0x2e prefix which maps to INAT_PFX_CS > > > > attribute in is_prefix_bad function. > > > > > > > > Also changing the uprobe syscall error when called out of uprobe > > > > trampoline to -EPROTO, so we are able to detect the fixed kernel. > > > > > > > > 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/ > > > > [2] https://lore.kernel.org/bpf/20260518104306.GU3102624@noisy.programming.kicks-ass.net/#t > > > > 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") > > > > Assisted-by: Codex:GPT-5.5 > > > > Signed-off-by: Jiri Olsa > > > > --- > > > > arch/x86/kernel/uprobes.c | 255 ++++++++++++++++++++++++++++---------- > > > > 1 file changed, 190 insertions(+), 65 deletions(-) > > > > > > > > > > [...] > > > > > > > @@ -943,13 +1026,31 @@ static int int3_update(struct arch_uprobe *auprobe, struct vm_area_struct *vma, > > > > smp_text_poke_sync_each_cpu(); > > > > > > > > /* > > > > - * Write first byte. > > > > + * 3) Restore bytes 1..4 of the original NOP while keeping byte 0 trapped > > > > + * and byte 5 as CALL: > > > > + * cc [2e 0f 1f 84] e8 d0 d1 d2 d3 > > > > + */ > > > > + ctx.expect = EXPECT_SWBP_OPTIMIZED; > > > > + err = uprobe_write(auprobe, vma, vaddr + 1, insn + 1, > > > > + LEA_INSN_SIZE - 1, verify_insn, > > > > + true /* is_register */, false /* do_update_ref_ctr */, > > > > > > tbh, it's quite subtle and non-obvious why is_register should be set > > > to true first two times (and especially that is_register and > > > do_update_ref_ctr are implicitly connected), not sure how to make it > > > cleaner, but maybe leave a short comment explaining this twice > > > register, once unregister sequence? > > > > ok, I came up with comment below > > > > thanks, > > jirka > > > > > > --- > > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c > > index de544516ea70..92449f34c005 100644 > > --- a/arch/x86/kernel/uprobes.c > > +++ b/arch/x86/kernel/uprobes.c > > @@ -1011,6 +1011,12 @@ static int int3_update_unoptimize(struct arch_uprobe *auprobe, struct vm_area_st > > int err; > > > > /* > > + * Note the first two uprobe_write calls use is_register=true, because they > > + * are intermediate patching states while the probe is still active. > > this doesn't really explain why is_register=true is the right one. It > actually doesn't matter as long as do_update_ref_ctr=true, isn't that > right? So maybe just to avoid a bit of confusion let's pass > is_register=false and do_update_ref_ctr=false, and in the comment > explain as you said that it's intermediate update and we don't want to > update refctr just yet until the very last step? apart from refctr update there's also different way the concerned page is managed, IIUC: with is_register=true we force to get exclusive anonymous page for the update (or pin the existing one) with is_register=false we try to zap the private anonymous page and return the mapping to the original page there are several comments on this in uprobe_write/__uprobe_write how about the update below jirka --- diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c index de544516ea70..09f5ff71227c 100644 --- a/arch/x86/kernel/uprobes.c +++ b/arch/x86/kernel/uprobes.c @@ -1011,6 +1011,16 @@ static int int3_update_unoptimize(struct arch_uprobe *auprobe, struct vm_area_st int err; /* + * Note the first two uprobe_write calls use is_register=true, because they + * are intermediate patching states while the probe is still active, so + * we force the exclusive anonymous page for the update. + * Also we use do_update_ref_ctr=false because refctr was already updated by + * the initial int3 install. + * + * The last uprobe_write to nop10 instruction is called with is_register=false + * and do_update_ref_ctr=true to trigger the refctr update and to instruct + * uprobe_write to zap the anonymous page if it now matches the file page. + * * 1) Initial optimized state: * 48 8d 64 24 80 e8 d0 d1 d2 d3 *