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@xxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
Link: https://lore.kernel.org/bpf/30a7b5d5-6726-1cc2-eaee-8da2828a9a9c@xxxxxxxxxx
Link: https://lore.kernel.org/bpf/20220314182042.71025-1-namhyung@xxxxxxxxxx
Based-on-patch-by: Eugene Loh <eugene.loh@xxxxxxxxxx>
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 7604e7d5438f..d14b10b85e51 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3009,8 +3009,8 @@ 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
@@ -4316,8 +4316,8 @@ 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
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 38bdfcd06f55..34725bfa1e97 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -176,7 +176,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
}
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;
@@ -187,9 +187,8 @@ get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
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
@@ -201,7 +200,7 @@ get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
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]);
}
@@ -218,27 +217,19 @@ static long __bpf_get_stackid(struct bpf_map *map,
{
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]);
@@ -295,8 +286,7 @@ 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);
- /* 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;
@@ -305,8 +295,12 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
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 */
@@ -397,7 +391,7 @@ 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)
{
- 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;
@@ -422,30 +416,28 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
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
