On Fri, Aug 29, 2025 at 10:29 AM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: [...] > > > > static long __bpf_get_stackid(struct bpf_map *map, > > - struct perf_callchain_entry *trace, u64 flags) > > + struct perf_callchain_entry *trace, u64 flags, u32 max_depth) > > { > > struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map); > > struct stack_map_bucket *bucket, *new_bucket, *old_bucket; > > @@ -263,6 +263,8 @@ static long __bpf_get_stackid(struct bpf_map *map, > > > > trace_nr = trace->nr - skip; > > trace_len = trace_nr * sizeof(u64); > > + trace_nr = min(trace_nr, max_depth - skip); > > + > > The patch might have fixed this particular syzbot repro > with OOB in stackmap-with-buildid case, > but above two line looks wrong. > trace_len is computed before being capped by max_depth. > So non-buildid case below is using > memcpy(new_bucket->data, ips, trace_len); > > so OOB is still there? +1 for this observation. We are calling __bpf_get_stackid() from two functions: bpf_get_stackid and bpf_get_stackid_pe. The check against max_depth is only needed from bpf_get_stackid_pe, so it is better to just check here. I have got the following on top of patch 1/2. This makes more sense to me. PS: The following also includes some clean up in __bpf_get_stack. I include those because it also uses stack_map_calculate_max_depth. Does this look better? Thanks, Song diff --git c/kernel/bpf/stackmap.c w/kernel/bpf/stackmap.c index 796cc105eacb..08554fb146e1 100644 --- c/kernel/bpf/stackmap.c +++ w/kernel/bpf/stackmap.c @@ -262,7 +262,7 @@ static long __bpf_get_stackid(struct bpf_map *map, return -EFAULT; trace_nr = trace->nr - skip; - trace_len = trace_nr * sizeof(u64); + ips = trace->ip + skip; hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0); id = hash & (smap->n_buckets - 1); @@ -297,6 +297,7 @@ static long __bpf_get_stackid(struct bpf_map *map, return -EEXIST; } } else { + trace_len = trace_nr * sizeof(u64); if (hash_matches && bucket->nr == trace_nr && memcmp(bucket->data, ips, trace_len) == 0) return id; @@ -322,19 +323,17 @@ static long __bpf_get_stackid(struct bpf_map *map, BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map, u64, flags) { - u32 max_depth = map->value_size / stack_map_data_size(map); - u32 skip = flags & BPF_F_SKIP_FIELD_MASK; + u32 elem_size = stack_map_data_size(map); bool user = flags & BPF_F_USER_STACK; struct perf_callchain_entry *trace; bool kernel = !user; + u32 max_depth; if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK | BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID))) return -EINVAL; - max_depth += skip; - if (max_depth > sysctl_perf_event_max_stack) - max_depth = sysctl_perf_event_max_stack; + max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags); trace = get_perf_callchain(regs, 0, kernel, user, max_depth, false, false); @@ -375,6 +374,7 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx, bool kernel, user; __u64 nr_kernel; int ret; + u32 elem_size, max_depth; /* perf_sample_data doesn't have callchain, use bpf_get_stackid */ if (!(event->attr.sample_type & PERF_SAMPLE_CALLCHAIN)) @@ -393,11 +393,12 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx, return -EFAULT; nr_kernel = count_kernel_ip(trace); - + elem_size = stack_map_data_size(map); if (kernel) { __u64 nr = trace->nr; - trace->nr = nr_kernel; + max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags); + trace->nr = min_t(u32, nr_kernel, max_depth); ret = __bpf_get_stackid(map, trace, flags); /* restore nr */ @@ -410,6 +411,8 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx, return -EFAULT; flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip; + max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags); + trace->nr = min_t(u32, trace->nr, max_depth); ret = __bpf_get_stackid(map, trace, flags); } return ret; @@ -428,7 +431,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, struct perf_callchain_entry *trace_in, void *buf, u32 size, u64 flags, bool may_fault) { - u32 trace_nr, copy_len, elem_size, num_elem, max_depth; + u32 trace_nr, copy_len, elem_size, max_depth; bool user_build_id = flags & BPF_F_USER_BUILD_ID; bool crosstask = task && task != current; u32 skip = flags & BPF_F_SKIP_FIELD_MASK; @@ -465,13 +468,15 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, if (may_fault) rcu_read_lock(); /* need RCU for perf's callchain below */ - if (trace_in) + if (trace_in) { trace = trace_in; - else if (kernel && task) + trace->nr = min_t(u32, trace->nr, max_depth); + } else if (kernel && task) { trace = get_callchain_entry_for_task(task, max_depth); - else + } else { trace = get_perf_callchain(regs, 0, kernel, user, max_depth, crosstask, false); + } if (unlikely(!trace) || trace->nr < skip) { if (may_fault) @@ -479,9 +484,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task, goto err_fault; } - num_elem = size / elem_size; trace_nr = trace->nr - skip; - trace_nr = (trace_nr <= num_elem) ? trace_nr : num_elem; copy_len = trace_nr * elem_size; ips = trace->ip + skip;