From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (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 D41AD368D59 for ; Fri, 22 May 2026 18:05:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779473114; cv=none; b=iCfVUqXCS1ihsdnZ9LC8WiNRNXv+GQkA2FF9GM3It104W8ImiV7PLmEEg+ak6pTuU/YEEo0OT8VRTj/w+Kchz4Jg5mYIVF+8JkAC0lxQmMChIu+sVNkCJCTfNpIgz+LW/3d1TOajm/G7y3R1InxUwkCtvasZaIEWly9w4ceTZko= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779473114; c=relaxed/simple; bh=Xj+OFoLuXhLvGMwXVVrB907QQVD1jZgT2IxH6x4cYa8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QpA1Uu45dKl66aDLZCx7pQZNjV98bic3/3PtxTXqPxgbOlusX3Y7cXceBWuvPTmHQiD/bPn1foN3cL9AV6uJ+oZTd4FyNTUzIYNazpDf3b+z1qSmMCgN3pDdBmn+w/19Zv2pMqw+fpL9EYURdRO2gruUyEkJlv6gukco/9zjFMg= 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=tDxAsecN; arc=none smtp.client-ip=95.215.58.171 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="tDxAsecN" Message-ID: <7b1a89e6-d473-4ef6-b8de-09c87bda7337@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779473099; 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=ZeYnfArxJw9f8aoySqJcF0s1ka9b0ozSHGV4Y5P7NuA=; b=tDxAsecNWd/wvcsBEhqa6y+cMvF8sH5V3F+pUxTPhELnPmQVZvrxWYSN6obmj2jF5ewnKp TDA37ceG159xddxfXFL7wCjVIrbw+QOSLO8/Ud1ZLFBLHFmlhO77wT9mCdaJByFXNw8Nho FaMiyqgusTiP3xzcKQavJf/OzTBPiqQ= Date: Fri, 22 May 2026 11:04:41 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v6 2/3] bpf: Avoid faultable build ID reads under mm locks To: Andrii Nakryiko Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Eduard Zingerman , Kumar Kartikeya Dwivedi , Puranjay Mohan , Shakeel Butt , Mykyta Yatsenko , bpf@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260521225022.2695755-1-ihor.solodrai@linux.dev> <20260521225022.2695755-3-ihor.solodrai@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Ihor Solodrai In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 5/22/26 10:42 AM, Andrii Nakryiko wrote: > On Thu, May 21, 2026 at 3:51 PM Ihor Solodrai wrote: >> >> Sleepable build ID parsing can block in __kernel_read() [1], so the >> stackmap sleepable path must not call it while holding mmap_lock or a >> per-VMA read lock. >> >> The issue and the fix are conceptually similar to a recent procfs >> patch [2]. >> >> Resolve each covered VMA with a stable read-side reference, preferring >> lock_vma_under_rcu() and falling back to mmap_read_trylock() only long >> enough to acquire the VMA read lock. Take a reference to the backing >> file, drop the VMA lock, and then parse the build ID through >> (sleepable) build_id_parse_file(). >> >> We have to use mmap_read_trylock() (and give up on failure) in this >> context because taking mmap_read_lock() is generally unsafe on code >> paths reachable from BPF programs [3], and may lead to deadlocks. >> >> [1] https://lore.kernel.org/all/20251218005818.614819-1-shakeel.butt@linux.dev/ >> [2] https://lore.kernel.org/all/20260128183232.2854138-1-andrii@kernel.org/ >> [3] https://lore.kernel.org/bpf/2895ecd8-df1e-4cc0-b9f9-aef893dc2360@linux.dev/ >> >> Fixes: d4dd9775ec24 ("bpf: wire up sleepable bpf_get_stack() and bpf_get_task_stack() helpers") >> Suggested-by: Puranjay Mohan >> Signed-off-by: Ihor Solodrai >> --- >> kernel/bpf/stackmap.c | 107 ++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 107 insertions(+) >> >> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c >> index 4c753e02c415..95336c0e8b56 100644 >> --- a/kernel/bpf/stackmap.c >> +++ b/kernel/bpf/stackmap.c >> @@ -9,6 +9,7 @@ >> #include >> #include >> #include >> +#include >> #include "percpu_freelist.h" >> #include "mmap_unlock_work.h" >> >> @@ -174,6 +175,107 @@ static inline void stack_map_build_id_set_valid(struct bpf_stack_build_id *id, >> memcpy(id->build_id, build_id, BUILD_ID_SIZE_MAX); >> } >> >> +struct stack_map_vma_lock { >> + bool vma_locked; >> + struct vm_area_struct *vma; >> + struct mm_struct *mm; >> +}; >> + >> +static struct vm_area_struct *stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip) >> +{ >> + struct mm_struct *mm = lock->mm; >> + struct vm_area_struct *vma; >> + >> + if (WARN_ON_ONCE(!mm)) >> + return NULL; >> + >> + vma = lock_vma_under_rcu(mm, ip); >> + if (vma) >> + goto vma_locked; >> + >> + /* >> + * Taking mmap_read_lock() is unsafe here, because the caller >> + * BPF program might already hold it, causing a deadlock. >> + */ >> + if (!mmap_read_trylock(mm)) >> + return NULL; >> + >> + vma = vma_lookup(mm, ip); >> + if (!vma) { >> + mmap_read_unlock(mm); >> + return NULL; >> + } > > As the code is written right now, this vma_lookup is futile if we > don't have CONFIG_PER_VMA_LOCK, no? We'll just unlock mmap and return > NULL regardless of this operation succeeding. So if that's the intent, > then starting from mmap_read_trylock() all the way to mmap_read_unlock > + return NULL should be under #ifdef CONFIG_PER_VMA_LOCK. > > But could it be that the intent was to set lock->vma_lock = false, > lock->vma=vma + return vma here if vma_lookup under mmap_lock > succeeded?... > > (oh, now scrolling further down the thread, seems like AIs picked up > on this as well, oh well) > >> + >> +#ifdef CONFIG_PER_VMA_LOCK >> + if (!vma_start_read_locked(vma)) { >> + mmap_read_unlock(mm); >> + return NULL; >> + } >> + mmap_read_unlock(mm); >> +#else >> + mmap_read_unlock(mm); >> + return NULL; > > see above, was this meant to be a "return vma"? > > This whole function is quite confusing, please help me understand how > it's supposed to work both with and without CONFIG_PER_VMA_LOCK. In v2 the stack_map_vma_lock could be holding either vma lock or mmap lock, in order to support CONFIG_PER_VMA_LOCK=n Then I tried testing it, and it turned out to set CONFIG_PER_VMA_LOCK=n you need to disable SMP, which seems like an unlikely kconfig: https://lore.kernel.org/bpf/bf91eb90-1b6a-4bad-a0e5-072f9dce7daa@linux.dev/ So since v3 I dropped attempt to support CONFIG_PER_VMA_LOCK=n by treating it the same way as failed mmap_read_trylock(), which is expressed by returning NULL from stack_map_lock_vma(). I guess you have a point in that the stackmap lock/unlock helpers can be refactored further. But with respect to CONFIG_PER_VMA_LOCK=n I don't see why we'd want to maintain additional complexity of mmap lock bookkeeping. > >> +#endif > >> +vma_locked: >> + lock->vma_locked = true; >> + lock->vma = vma; >> + return vma; >> +} >> + >> +static void stack_map_unlock_vma(struct stack_map_vma_lock *lock) >> +{ >> + struct vm_area_struct *vma = lock->vma; >> + >> + if (lock->vma_locked) { >> + if (WARN_ON_ONCE(!vma)) >> + goto out; > > this should never ever happen, it's a bug, we shouldn't warn on it, we > should make sure we never ever have vma_locked set to true if there is > no vma > > the whole pattern of returning NULL from stack_map_lock_vma() and > still holding lock (i.e., allowing and expecting to call > stack_map_unlock_vma) seems confusing and error-prone, tbh The idea of stack_map_lock_vma() was that it may fail like trylock, and it is safe to call corresponding unlock(), even if lock isn't held. This allows to have an unlock() on all paths after a lock() in the code, instead of "if locked" or "if null" checks. > > why not simplify it to "if we got vma and locked it -> we return > non-NULL vma and lock is held, otherwise no lock is held (because > why?)" I'll try that. > > pw-bot: cr > > >> + vma_end_read(vma); >> + } >> +out: >> + lock->vma_locked = false; >> + lock->vma = NULL; >> +} >> + >> +static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *id_offs, >> + u32 trace_nr) >> +{ >> + struct mm_struct *mm = current->mm; >> + struct stack_map_vma_lock lock = { >> + .vma_locked = false, >> + .vma = NULL, >> + .mm = mm, >> + }; >> + struct vm_area_struct *vma; >> + struct file *file; >> + u64 offset; >> + u64 ip; >> + >> + for (u32 i = 0; i < trace_nr; i++) { >> + ip = READ_ONCE(id_offs[i].ip); >> + >> + vma = stack_map_lock_vma(&lock, ip); >> + if (!vma || !vma->vm_file) { >> + stack_map_build_id_set_ip(&id_offs[i]); >> + stack_map_unlock_vma(&lock); > > see above, I find this confusing to need to call unlock_vma if there > is no vma in the first place... > >> + continue; >> + } >> + >> + file = get_file(vma->vm_file); >> + offset = stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip); >> + stack_map_unlock_vma(&lock); >> + >> + /* build_id_parse_file() may block on filesystem reads */ >> + if (build_id_parse_file(file, id_offs[i].build_id, NULL)) { >> + stack_map_build_id_set_ip(&id_offs[i]); >> + fput(file); >> + continue; >> + } >> + fput(file); >> + >> + stack_map_build_id_set_valid(&id_offs[i], offset, id_offs[i].build_id); > > what about > > if (build_id_parse_file(...)) > stack_map_build_id_set_ip(...); /* no build id, parsing failed */ > else > stack_map_build_id_set_valid(...); > > fput(file); > > ? > >> + } >> +} >> + >> /* >> * Expects all id_offs[i].ip values to be set to correct initial IPs. >> * They will be subsequently: >> @@ -194,6 +296,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs, >> const unsigned char *prev_build_id; >> int i; >> >> + if (may_fault && has_user_ctx) { >> + stack_map_get_build_id_offset_sleepable(id_offs, trace_nr); >> + return; >> + } >> + >> /* If the irq_work is in use, fall back to report ips. Same >> * fallback is used for kernel stack (!user) on a stackmap with >> * build_id. >> -- >> 2.54.0 >>