From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr1-f51.google.com (mail-wr1-f51.google.com [209.85.221.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 032F72116F4 for ; Tue, 4 Nov 2025 09:27:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762248446; cv=none; b=cZQBTz1+aIOh0eQ0WprvVD1LCJGJ3mpvktBI2WQnzYQmbcaoN20zPfH41CDZWOkMjCiiw6AZw3Bj4X4tIhpgJwFLFCDPJEikEeiPB2MNje9inJAw3B+3jeC32G5/6lCI9/4Soloemy0OisxsceAjBymmpC8cN+wkln5+ppZSMkY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762248446; c=relaxed/simple; bh=VIL05nHFT4N6JAG/H2iMRz7OrtDZbp13GR+Lkr9m0m4=; h=From:Date:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Kee3WqaYOfu+aABlv9FGBOZZ+pLVG2RKZYP4SJgGWzhw1JnGycdPInF0EZ8Nbq78EPeSYEl8YqgU+lsn0OHYw0Z9w4naOU0susAsCrsvGAsj9YyNgGTg66fLwUzz7pk8lg8Q0RKY1HJy7XA4bmo3st8schTS/Wv7qQ4osmomwQ8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=K4IhtWMC; arc=none smtp.client-ip=209.85.221.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="K4IhtWMC" Received: by mail-wr1-f51.google.com with SMTP id ffacd0b85a97d-429c7869704so3315462f8f.2 for ; Tue, 04 Nov 2025 01:27:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1762248443; x=1762853243; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:date:from:from:to:cc:subject:date:message-id:reply-to; bh=oAiC86qBA7A39iWQR2bMUkqXVWA4G10ZvXba/KKkMyI=; b=K4IhtWMCtKLxMSMUCxBB4qWgLuSWTpzCT12alYZRcv4KDgzm7rz/ezkL79TgPc9cdN X4S/9WQThRupWvuEN5VYqQXHXyxoHHsIL1+nPrXv8g8PRmkJhC8ylBG5mPWthm6And69 q/YHnxLQVkrvTkW9n2AuTGoamQqxtp/yvkHje++WYSzQ4YnWg9pvZJvdj69kWA5GcGYE oSOCIyJ4h9R9gABKW7tCkU3fu8V4G6KQpoxNKkf78akoCNLmOexow4EHR9Nn3jwc500X cDBFf/TffiDhObbufEB9KNMDhiuJURctJeXoRITDpwlwhDx8CsADckLs0T8t8UY8f0a/ +sig== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1762248443; x=1762853243; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:date:from:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=oAiC86qBA7A39iWQR2bMUkqXVWA4G10ZvXba/KKkMyI=; b=gDJt45vFRMO4Q/L2Jr9BxrSJQ35XM/3NLv2pvktx1MW4grFUPUtQvMyCDFlG8Dk72u XVKdOahvSEmjHEUf3H9RWMgZV05Ici1we5a0tdu9AiMwjnCfwGlCV46/S+WsyIo5lzOL f7nl2df+WJcKfqEmM1zPtO1EOhMZh5CZADpRQZv6sJHGMWttmWJzG8fs9f/d3jalY+/Z jLAHpFvuqCl9eFTseV2LkaW8JNngz31KPVKEaLQddnIoSitzexNBTpcE3vZfaazLr46l 0xn42j7brG9ZMgHm0izOcRN84CDjdtlpIGGih2HHzzuL6O2b4ksW+f2QVRn/dQ/n1suy yowg== X-Forwarded-Encrypted: i=1; AJvYcCU/W8xJWH+guE4OlA/JsY4QTlDOFVmwE6xmWjYD8ZrwD7B+yCd2APFhiGOPcdGeZTDa794=@vger.kernel.org X-Gm-Message-State: AOJu0YwUHMdYekp2BNbNIYSx5xRUaRS5vmGTa0Amll+GUdvE1QJnA1S/ EPJcUXiSBH0i4HGG4/2TNbWeM4f5lOEnNQjFLV/8nAEjGm8BSj8Q+ttA X-Gm-Gg: ASbGncsLrL0KmOD9KRIm0AXz4N5AF03PDp4v/ybtF/ffp+Ao5amDP0RGr+4vXSfCXtn TL1zEI9Lg3/XxgqPBDQOSa/m0WlN9pVIgiLWBN2wQlxMdUmXNQ+z6ZMNJMkJJ59GpeDMv3swyec ptvft18BKFV3lmVx/wl03pNuGhT7Pzr4jY7+MD+XE4uX0lboEBWhJLAZ1AubyuJdlVRcnzqynFF xUH5SqEWVsQJ4c5zVu/rcEmU06WtMUHk7zhPyaldqmbGeVEqoWm5RYSQ9oeg007eMb1idl939MJ XUmG37iUUH3cdLfAbtjhgS0eHzyVqTNVNx1bnOc9c92b3JzQLXOjPtNlPhnR9iBOkpRYH8G7ik8 tX77wtIhhonv7MTN08oK910vm57xbV3LCnojBf7KDHeULTtI3Sw== X-Google-Smtp-Source: AGHT+IG1Iz02Do8vk/OkUauhbzeE1XTOlKai6jAIc/dx9gUu7tjggesIQloqaLtajEfFQGe39i3EMQ== X-Received: by 2002:a05:6000:400d:b0:3f7:b7ac:f3d2 with SMTP id ffacd0b85a97d-429bd6be4e8mr13062531f8f.43.1762248443219; Tue, 04 Nov 2025 01:27:23 -0800 (PST) Received: from krava ([2a02:8308:a00c:e200::b44f]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429dc18f41bsm3393650f8f.9.2025.11.04.01.27.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Nov 2025 01:27:22 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Tue, 4 Nov 2025 10:27:20 +0100 To: bot+bpf-ci@kernel.org Cc: mhiramat@kernel.org, rostedt@goodmis.org, jpoimboe@kernel.org, peterz@infradead.org, bpf@vger.kernel.org, linux-trace-kernel@vger.kernel.org, x86@kernel.org, yhs@fb.com, songliubraving@fb.com, andrii@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: [PATCHv2 2/4] x86/fgraph,bpf: Fix stack ORC unwind from kprobe_multi return probe Message-ID: References: <20251103220924.36371-3-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 Mon, Nov 03, 2025 at 11:47:37PM +0000, bot+bpf-ci@kernel.org wrote: > > diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h > > index 93156ac4f..b08c95872 100644 > > --- a/arch/x86/include/asm/ftrace.h > > +++ b/arch/x86/include/asm/ftrace.h > > @@ -56,6 +56,11 @@ arch_ftrace_get_regs(struct ftrace_regs *fregs) > > return &arch_ftrace_regs(fregs)->regs; > > } > > > > +#define arch_ftrace_partial_regs(regs) do { \ > > + regs->flags &= ~X86_EFLAGS_FIXED; \ > > + regs->cs = __KERNEL_CS; \ > > +} while (0) > > + > > Can arch_ftrace_partial_regs read uninitialized regs->flags? > > In return_to_handler, the ftrace_regs structure is allocated on the > stack but only a few fields are initialized (RAX, RDX, RBP, RSP). > The EFLAGS/flags field is never written before > arch_ftrace_partial_regs reads it: > > arch/x86/kernel/ftrace_64.S return_to_handler: > subq $(FRAME_SIZE), %rsp # Allocate ftrace_regs on stack > movq %rax, RAX(%rsp) > movq %rdx, RDX(%rsp) > movq %rbp, RBP(%rsp) > movq %rsp, RSP(%rsp) > # No initialization of EFLAGS field > > Then ftrace_partial_regs()->arch_ftrace_partial_regs() executes: > regs->flags &= ~X86_EFLAGS_FIXED; > > This reads the uninitialized flags field. Stack allocations contain > garbage, so regs->flags will have whatever data was previously on > the stack. The &= operation produces undefined results when operating > on uninitialized memory. > > For comparison, ftrace_regs_caller explicitly initializes EFLAGS: > movq MCOUNT_REG_SIZE(%rsp), %rcx > movq %rcx, EFLAGS(%rsp) > > Should return_to_handler initialize regs->flags to 0 (or another > appropriate value) before arch_ftrace_partial_regs modifies it? yes, that was in the initial version [1] but Stephen did not like that, so I moved the flags init to later where it needs to take into account other callers (that have meaningful flags) and just zeros the X86_EFLAGS_FIXED, so we go through !perf_hw_regs leg in perf_callchain_kernel jirka [1] https://lore.kernel.org/bpf/aObSyt3qOnS_BMcy@krava/