From: Namhyung Kim namhyung@kernel.org
commit ee2a098851bfbe8bcdd964c0121f4246f00ff41e upstream.
Let's say that the caller has storage for num_elem stack frames. Then, the BPF stack helper functions walk the stack for only num_elem frames. This means that if skip > 0, one keeps only 'num_elem - skip' frames.
This is because it sets init_nr in the perf_callchain_entry to the end of the buffer to save num_elem entries only. I believe it was because the perf callchain code unwound the stack frames until it reached the global max size (sysctl_perf_event_max_stack).
However it now has perf_callchain_entry_ctx.max_stack to limit the iteration locally. This simplifies the code to handle init_nr in the BPF callstack entries and removes the confusion with the perf_event's __PERF_SAMPLE_CALLCHAIN_EARLY which sets init_nr to 0.
Also change the comment on bpf_get_stack() in the header file to be more explicit what the return value means.
Fixes: c195651e565a ("bpf: add bpf_get_stack helper") Signed-off-by: Namhyung Kim namhyung@kernel.org Signed-off-by: Alexei Starovoitov ast@kernel.org Acked-by: Yonghong Song yhs@fb.com Link: https://lore.kernel.org/bpf/30a7b5d5-6726-1cc2-eaee-8da2828a9a9c@oracle.com Link: https://lore.kernel.org/bpf/20220314182042.71025-1-namhyung@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
Based-on-patch-by: Eugene Loh eugene.loh@oracle.com --- include/uapi/linux/bpf.h | 8 +++--- kernel/bpf/stackmap.c | 56 ++++++++++++++++++++--------------------------- 2 files changed, 28 insertions(+), 36 deletions(-)
--- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -2965,8 +2965,8 @@ union bpf_attr { * * # sysctl kernel.perf_event_max_stack=<new value> * Return - * A non-negative value equal to or less than *size* on success, - * or a negative error in case of failure. + * The non-negative copied *buf* length equal to or less than + * *size* on success, or a negative error in case of failure. * * long bpf_skb_load_bytes_relative(const void *skb, u32 offset, void *to, u32 len, u32 start_header) * Description @@ -4269,8 +4269,8 @@ union bpf_attr { * * # sysctl kernel.perf_event_max_stack=<new value> * Return - * A non-negative value equal to or less than *size* on success, - * or a negative error in case of failure. + * The non-negative copied *buf* length equal to or less than + * *size* on success, or a negative error in case of failure. * * long bpf_load_hdr_opt(struct bpf_sock_ops *skops, void *searchby_res, u32 len, u64 flags) * Description --- a/kernel/bpf/stackmap.c +++ b/kernel/bpf/stackmap.c @@ -219,7 +219,7 @@ static void stack_map_get_build_id_offse }
static struct perf_callchain_entry * -get_callchain_entry_for_task(struct task_struct *task, u32 init_nr) +get_callchain_entry_for_task(struct task_struct *task, u32 max_depth) { #ifdef CONFIG_STACKTRACE struct perf_callchain_entry *entry; @@ -230,9 +230,8 @@ get_callchain_entry_for_task(struct task if (!entry) return NULL;
- entry->nr = init_nr + - stack_trace_save_tsk(task, (unsigned long *)(entry->ip + init_nr), - sysctl_perf_event_max_stack - init_nr, 0); + entry->nr = stack_trace_save_tsk(task, (unsigned long *)entry->ip, + max_depth, 0);
/* stack_trace_save_tsk() works on unsigned long array, while * perf_callchain_entry uses u64 array. For 32-bit systems, it is @@ -244,7 +243,7 @@ get_callchain_entry_for_task(struct task int i;
/* copy data from the end to avoid using extra buffer */ - for (i = entry->nr - 1; i >= (int)init_nr; i--) + for (i = entry->nr - 1; i >= 0; i--) to[i] = (u64)(from[i]); }
@@ -261,27 +260,19 @@ static long __bpf_get_stackid(struct bpf { struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map); struct stack_map_bucket *bucket, *new_bucket, *old_bucket; - u32 max_depth = map->value_size / stack_map_data_size(map); - /* stack_map_alloc() checks that max_depth <= sysctl_perf_event_max_stack */ - u32 init_nr = sysctl_perf_event_max_stack - max_depth; u32 skip = flags & BPF_F_SKIP_FIELD_MASK; u32 hash, id, trace_nr, trace_len; bool user = flags & BPF_F_USER_STACK; u64 *ips; bool hash_matches;
- /* get_perf_callchain() guarantees that trace->nr >= init_nr - * and trace-nr <= sysctl_perf_event_max_stack, so trace_nr <= max_depth - */ - trace_nr = trace->nr - init_nr; - - if (trace_nr <= skip) + if (trace->nr <= skip) /* skipping more than usable stack trace */ return -EFAULT;
- trace_nr -= skip; + trace_nr = trace->nr - skip; trace_len = trace_nr * sizeof(u64); - ips = trace->ip + skip + init_nr; + ips = trace->ip + skip; hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0); id = hash & (smap->n_buckets - 1); bucket = READ_ONCE(smap->buckets[id]); @@ -338,8 +329,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_re u64, flags) { u32 max_depth = map->value_size / stack_map_data_size(map); - /* stack_map_alloc() checks that max_depth <= sysctl_perf_event_max_stack */ - u32 init_nr = sysctl_perf_event_max_stack - max_depth; + u32 skip = flags & BPF_F_SKIP_FIELD_MASK; bool user = flags & BPF_F_USER_STACK; struct perf_callchain_entry *trace; bool kernel = !user; @@ -348,8 +338,12 @@ BPF_CALL_3(bpf_get_stackid, struct pt_re BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID))) return -EINVAL;
- trace = get_perf_callchain(regs, init_nr, kernel, user, - sysctl_perf_event_max_stack, false, false); + max_depth += skip; + if (max_depth > sysctl_perf_event_max_stack) + max_depth = sysctl_perf_event_max_stack; + + trace = get_perf_callchain(regs, 0, kernel, user, max_depth, + false, false);
if (unlikely(!trace)) /* couldn't fetch the stack trace */ @@ -440,7 +434,7 @@ static long __bpf_get_stack(struct pt_re struct perf_callchain_entry *trace_in, void *buf, u32 size, u64 flags) { - u32 init_nr, trace_nr, copy_len, elem_size, num_elem; + u32 trace_nr, copy_len, elem_size, num_elem, max_depth; bool user_build_id = flags & BPF_F_USER_BUILD_ID; u32 skip = flags & BPF_F_SKIP_FIELD_MASK; bool user = flags & BPF_F_USER_STACK; @@ -465,30 +459,28 @@ static long __bpf_get_stack(struct pt_re goto err_fault;
num_elem = size / elem_size; - if (sysctl_perf_event_max_stack < num_elem) - init_nr = 0; - else - init_nr = sysctl_perf_event_max_stack - num_elem; + max_depth = num_elem + skip; + if (sysctl_perf_event_max_stack < max_depth) + max_depth = sysctl_perf_event_max_stack;
if (trace_in) trace = trace_in; else if (kernel && task) - trace = get_callchain_entry_for_task(task, init_nr); + trace = get_callchain_entry_for_task(task, max_depth); else - trace = get_perf_callchain(regs, init_nr, kernel, user, - sysctl_perf_event_max_stack, + trace = get_perf_callchain(regs, 0, kernel, user, max_depth, false, false); if (unlikely(!trace)) goto err_fault;
- trace_nr = trace->nr - init_nr; - if (trace_nr < skip) + if (trace->nr < skip) goto err_fault;
- trace_nr -= skip; + 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 + init_nr; + + ips = trace->ip + skip; if (user && user_build_id) stack_map_get_build_id_offset(buf, ips, trace_nr, user); else