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bpf: Adjust BPF stack helper functions to accommodate skip > 0
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commit ee2a098 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: c195651 ("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>
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@namhyung @gregkh
namhyung authored and gregkh committed Apr 8, 2022
1 parent 8ed35dc commit 6c4f243
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Showing 2 changed files with 28 additions and 36 deletions.
8 changes: 4 additions & 4 deletions include/uapi/linux/bpf.h
View file
Expand Up @@ -2975,8 +2975,8 @@ 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
Expand Down Expand Up @@ -4279,8 +4279,8 @@ 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
Expand Down
56 changes: 24 additions & 32 deletions kernel/bpf/stackmap.c
View file
Expand Up @@ -166,7 +166,7 @@ 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;
Expand All @@ -177,9 +177,8 @@ 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
Expand All @@ -191,7 +190,7 @@ 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]);
}

Expand All @@ -208,27 +207,19 @@ 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]);
Expand Down Expand Up @@ -285,8 +276,7 @@ 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;
Expand All @@ -295,8 +285,12 @@ 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 */
Expand Down Expand Up @@ -387,7 +381,7 @@ 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;
Expand All @@ -412,30 +406,28 @@ 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
Expand Down

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