From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-181.mta1.migadu.com (out-181.mta1.migadu.com [95.215.58.181]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E28893624CB for ; Tue, 28 Apr 2026 20:54:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.181 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777409671; cv=none; b=qViTziZ4SPuIUbZMkViOPa/d1Sg+L5P7sKVkxTl7g9wRvLPfY7jI8AXs1yoZvVeTV/6kSYC+ePqYGqL0J6AVFJbDeOxTlAvw2c1FSCnpxR6ySgv42eez2Y6/yBIO2QTv81kmjAXBGvAw5o+KdATmbSeUJAAiRWTySJJXh8TU+3E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777409671; c=relaxed/simple; bh=F3MkMNEDXiJ9N4b+H9WtSz3Chu9Wi6FA4WsPi1oRhcM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JCK03f8YquBQvRRYBdtXOGh9JpQo5I6Pu5kPxjVoYBHD4qhCyqJY32v6/hEu66gPT8zSvB8rPvRLSVV2mwDPhShsU3AzBETexIQXNkjiJmNnIQYQRzZYldO2aPWCLwaGnWvZDDqOeE4lxEZ8lXSutdfwuzFBYvPNoOLqnk2cbjo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=fNxGPUxH; arc=none smtp.client-ip=95.215.58.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="fNxGPUxH" Message-ID: <8c6455e1-94f5-42e5-8d27-66787dec878f@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1777409667; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=+CuKGev6BLhfF7EnMPMhetPzsqE/BpqDNIPe4P7qFl8=; b=fNxGPUxHLCkBLTX0lWJHc92rsqP3zwEp7LV5blEPvoYDbWcTk2GOBqxPxwZ5mXYzYXHQjY TMvIMR7L+9O8b1b1+Ut/3IV75wjr4DwL9dzbERNp58FLbo7MgiqsZqY9YjJWIsWLsYSzHV 7pO0HnNDeauZVXaCmj1v0YPnjmMq6Oo= Date: Tue, 28 Apr 2026 21:54:21 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next 02/18] bpf: Add precision marking and backtracking for stack argument slots Content-Language: en-GB To: Eduard Zingerman , bpf@vger.kernel.org Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , "Jose E . Marchesi" , kernel-team@fb.com, Martin KaFai Lau References: <20260424171433.2034470-1-yonghong.song@linux.dev> <20260424171443.2034958-1-yonghong.song@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 4/28/26 9:46 AM, Eduard Zingerman wrote: > On Fri, 2026-04-24 at 10:14 -0700, Yonghong Song wrote: >> Extend the precision marking and backtracking infrastructure to >> support stack argument slots (r11-based accesses). Without this, >> precision demands for scalar values passed through stack arguments >> are silently dropped, which could allow the verifier to incorrectly >> prune states with different constant values in stack arg slots. >> >> INSN_F_STACK_ARG_ACCESS is encoded as INSN_F_STACK_ACCESS | >> INSN_F_DST_REG_STACK (BIT(9) | BIT(10)). This is safe because >> INSN_F_STACK_ACCESS is only used for ST/STX/LDX insns while >> INSN_F_DST_REG_STACK is only used for JMP insns — they never appear >> on the same instruction. This keeps the total within the 12-bit >> jmp_history flags budget. >> >> Three components are added: >> >> 1. Jump history recording for stack arg accesses: >> - check_stack_arg_write() records INSN_F_STACK_ARG_ACCESS for >> outgoing stores. >> - check_stack_arg_read() records INSN_F_STACK_ARG_ACCESS for >> incoming loads. >> >> 2. backtrack_insn() handling: >> - BPF_LDX: when backtracking through an incoming stack arg load, >> transfer precision demand from the destination register to the >> stack arg slot mask. >> - BPF_STX/BPF_ST: when backtracking through an outgoing stack arg >> store, transfer precision demand from the stack arg slot to the >> source register. >> - Call boundary: when exiting a callee back to the caller, >> propagate the callee's incoming stack arg precision bits to the >> caller's outgoing stack arg slots. The slot index maps directly >> (slot i in callee corresponds to slot i in caller) since the >> caller's stack_arg_regs only contains outgoing slots. >> >> 3. bpf_mark_chain_precision() state walking: >> - When iterating parent states, mark stack_arg_regs[spi].precise >> for slots that have pending precision demand. >> >> Signed-off-by: Yonghong Song >> --- > Acked-by: Eduard Zingerman > >> include/linux/bpf_verifier.h | 13 ++++++++ >> kernel/bpf/backtrack.c | 61 ++++++++++++++++++++++++++++++++++-- >> kernel/bpf/verifier.c | 30 +++++++++++++++--- >> 3 files changed, 98 insertions(+), 6 deletions(-) >> >> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h >> index 2cc349d7fc17..735f33ad3db7 100644 >> --- a/include/linux/bpf_verifier.h >> +++ b/include/linux/bpf_verifier.h >> @@ -393,6 +393,13 @@ enum { >> INSN_F_SPI_SHIFT = 3, /* shifted 3 bits to the left */ >> >> INSN_F_STACK_ACCESS = BIT(9), >> + /* >> + * INSN_F_STACK_ARG_ACCESS uses INSN_F_STACK_ACCESS | INSN_F_DST_REG_STACK. >> + * This is safe because INSN_F_DST_REG_STACK is only used for JMP insns >> + * while INSN_F_STACK_ACCESS is only used for ST/STX/LDX insns — they >> + * never appear on the same instruction. >> + */ >> + INSN_F_STACK_ARG_ACCESS = BIT(9) | BIT(10), > Tbh, I'd split bpf_jmp_history_entry like this: > > struct bpf_jmp_history_entry { > u32 idx:20; > u32 frame:3; > u32 spi:6; > /* insn idx can't be bigger than 1 million */ > u32 prev_idx : 20; > /* special INSN_F_xxx flags */ > u32 flags : 12; > /* additional registers that need precision tracking when this > * jump is backtracked, vector of six 10-bit records > */ > u64 linked_regs; > }; Good point, let me try. > > [...] > >> @@ -453,9 +498,10 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx, >> bt_stack_mask(bt)); >> return -EFAULT; >> } >> - /* clear r1-r5 in callback subprog's mask */ >> + /* clear r1-r5 and stack arg slots in callback subprog's mask */ >> for (i = BPF_REG_1; i <= BPF_REG_5; i++) >> bt_clear_reg(bt, i); >> + bt->stack_arg_masks[bt->frame] = 0; > Nit: I think having these set at this point is a bug condition. This one is a leftover from early patch and I didn't remove it. I just tested again. this is indeed not needed. Will remove. > >> if (bt_subprog_exit(bt)) >> return -EFAULT; >> return 0; > [...] > >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > [...] > >> static int check_outgoing_stack_args(struct bpf_verifier_env *env, struct bpf_func_state *caller, >> @@ -7269,8 +7285,14 @@ static int check_mem_size_reg(struct bpf_verifier_env *env, >> } >> err = check_helper_mem_access(env, mem_reg, mem_argno, size_reg->umax_value, >> access_type, zero_size_allowed, meta); >> - if (!err) >> - err = mark_chain_precision(env, reg_from_argno(size_argno)); >> + if (!err) { >> + int regno = reg_from_argno(size_argno); >> + >> + if (regno >= 0) >> + err = mark_chain_precision(env, regno); >> + else >> + err = mark_stack_arg_precision(env, arg_from_argno(size_argno) - 1); >> + } > Nit: maybe make this a utility function, e.g. mark_arg_precision(env, argno) ? Sure, will do. > >> return err; >> } >>