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Mon, 18 May 2026 01:31:53 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Mon, 18 May 2026 10:31:51 +0200 To: Jiri Olsa Cc: sashiko-reviews@lists.linux.dev, bpf@vger.kernel.org Subject: Re: [PATCH 1/7] uprobes/x86: Move optimized uprobe from nop5 to nop10 Message-ID: References: <20260514135342.22130-2-jolsa@kernel.org> <20260514200545.C672EC2BCB3@smtp.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 Sun, May 17, 2026 at 01:42:50PM +0200, Jiri Olsa wrote: SNIP > > > Is there a similar issue for forked children that inherit the optimized > > > 10-byte sequence? > > > > > > During fork, uprobe_mmap() installs a breakpoint in the child by writing > > > 0xCC to the first byte, changing the instruction from 48 8d... to cc 8d... > > > > > > If the uprobe is unregistered before the child hits and re-optimizes it, > > > __is_optimized() will return false because is_lea_insn() strictly expects > > > the first byte to be 0x48: > > > > > > arch/x86/kernel/uprobes.c:is_lea_insn() { > > > return !memcmp(insn, lea_rsp, LEA_INSN_SIZE); > > > } > > > > > > The fallback check for the re-attached probe on top of jmp8 also fails > > > because insn[1] is 0x8d, not OPT_JMP8_OFFSET. > > > > > > Could set_orig_insn() then fall back to writing just the first byte of > > > the original instruction over the 0xcc, again leaving 66 8d 64 24 80 e8 ... > > > and silently truncating the child's stack pointer? > > > > nice.. maybe we can skip the install_breakpoint call in uprobe_mmap > > for optimized probes.. will check > > I think we need to dup uprobe trampolines on fork like below or just allow it to be copied on fork jirka --- diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c index 4d91930aa792..e0067d1b6242 100644 --- a/arch/x86/kernel/uprobes.c +++ b/arch/x86/kernel/uprobes.c @@ -719,7 +719,7 @@ static struct uprobe_trampoline *create_uprobe_trampoline(unsigned long vaddr) tramp->vaddr = vaddr; vma = _install_special_mapping(mm, tramp->vaddr, PAGE_SIZE, - VM_READ|VM_EXEC|VM_MAYEXEC|VM_MAYREAD|VM_DONTCOPY|VM_IO, + VM_READ|VM_EXEC|VM_MAYEXEC|VM_MAYREAD|VM_IO, &tramp_mapping); if (IS_ERR(vma)) { kfree(tramp);