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([2a02:8109:a307:d900:a778:a387:b2af:188b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43d63de2a69sm49233f8f.4.2026.04.09.09.34.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Apr 2026 09:34:23 -0700 (PDT) Message-ID: <8b2cf5ca-182e-4eb2-ab90-8fab9d81f6e1@gmail.com> Date: Thu, 9 Apr 2026 17:34:19 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Mykyta Yatsenko Subject: Re: [PATCH bpf v2 2/2] bpf: Avoid faultable build ID reads under mm locks To: Ihor Solodrai , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Song Liu Cc: Puranjay Mohan , Shakeel Butt , bpf@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260409010604.1439087-1-ihor.solodrai@linux.dev> <20260409010604.1439087-3-ihor.solodrai@linux.dev> Content-Language: en-US In-Reply-To: <20260409010604.1439087-3-ihor.solodrai@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 4/9/26 2:06 AM, 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_lock() only long meganit: falling back to mmap_read_trylock()? > 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(). > > [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/ > > Fixes: 777a8560fd29 ("lib/buildid: use __kernel_read() for sleepable context") > Assisted-by: Codex:gpt-5.4 > Suggested-by: Puranjay Mohan > Signed-off-by: Ihor Solodrai > --- > kernel/bpf/stackmap.c | 139 ++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 139 insertions(+) > > diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c > index 4ef0fd06cea5..de3d89e20a1e 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" > > @@ -158,6 +159,139 @@ static inline void stack_map_build_id_set_ip(struct bpf_stack_build_id *id) > memset(id->build_id, 0, BUILD_ID_SIZE_MAX); > } > > +enum stack_map_vma_lock_state { > + STACK_MAP_LOCKED_NONE = 0, > + STACK_MAP_LOCKED_VMA, > + STACK_MAP_LOCKED_MMAP, > +}; > + > +struct stack_map_vma_lock { > + enum stack_map_vma_lock_state state; > + 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; > + > + if (!mmap_read_trylock(mm)) > + return NULL; > + > + vma = vma_lookup(mm, ip); > + if (!vma) { > + mmap_read_unlock(mm); > + return NULL; > + } > + > +#ifdef CONFIG_PER_VMA_LOCK > + if (!vma_start_read_locked(vma)) { > + mmap_read_unlock(mm); > + return NULL; > + } > + mmap_read_unlock(mm); > +#else > + lock->state = STACK_MAP_LOCKED_MMAP; > + lock->vma = vma; > + return vma; > +#endif > + > +vma_locked: > + lock->state = STACK_MAP_LOCKED_VMA; > + lock->vma = vma; > + return vma; > +} > + > +static void stack_map_unlock_vma(struct stack_map_vma_lock *lock) > +{ > + struct vm_area_struct *vma = lock->vma; > + struct mm_struct *mm = lock->mm; > + > + switch (lock->state) { > + case STACK_MAP_LOCKED_VMA: > + if (WARN_ON_ONCE(!vma)) > + break; > + vma_end_read(vma); > + break; > + case STACK_MAP_LOCKED_MMAP: > + if (WARN_ON_ONCE(!mm)) > + break; > + mmap_read_unlock(mm); > + break; > + default: > + break; > + } > + > + lock->state = STACK_MAP_LOCKED_NONE; > + 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 = { > + .state = STACK_MAP_LOCKED_NONE, > + .vma = NULL, > + .mm = mm, > + }; > + struct file *file, *prev_file = NULL; > + unsigned long vm_pgoff, vm_start; > + struct vm_area_struct *vma; > + const char *prev_build_id; > + u64 ip; > + > + for (u32 i = 0; i < trace_nr; i++) { > + ip = READ_ONCE(id_offs[i].ip); I'm not sure if I understand why READ_ONCE is necessary here. > + 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); > + continue; > + } > + > + file = vma->vm_file; > + vm_pgoff = vma->vm_pgoff; > + vm_start = vma->vm_start; > + > + if (file == prev_file) { What if instead of caching prev_file, we cache vm_start and vm_end, and if the next IP is in range, reuse previous build id. This should optimize this code further, avoiding locks on the vma used on previous iteration. > + memcpy(id_offs[i].build_id, prev_build_id, BUILD_ID_SIZE_MAX); > + stack_map_unlock_vma(&lock); > + goto build_id_valid; > + } > + > + file = get_file(file); > + 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; > + } > + > + if (prev_file) > + fput(prev_file); > + prev_file = file; > + prev_build_id = id_offs[i].build_id; > + > +build_id_valid: > + id_offs[i].offset = (vm_pgoff << PAGE_SHIFT) + ip - vm_start; > + id_offs[i].status = BPF_STACK_BUILD_ID_VALID; > + } > + > + if (prev_file) > + fput(prev_file); > +} > + > /* > * Expects all id_offs[i].ip values to be set to correct initial IPs. > * They will be subsequently: > @@ -178,6 +312,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs, > const 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.