[PATCH v5 bpf-next 08/10] bpf: decouple stack_map_get_build_id_offset() from perf_callchain_entry

Andrii Nakryiko
2024-08-13T00:30:07Z

Change stack_map_get_build_id_offset() which is used to convert stack
trace IP addresses into build ID+offset pairs. Right now this function
accepts an array of u64s as an input, and uses array of
struct bpf_stack_build_id as an output.

This is problematic because u64 array is coming from
perf_callchain_entry, which is (non-sleepable) RCU protected, so once we
allows sleepable build ID fetching, this all breaks down.

But its actually pretty easy to make stack_map_get_build_id_offset()
works with array of struct bpf_stack_build_id as both input and output.
Which is what this patch is doing, eliminating the dependency on
perf_callchain_entry. We require caller to fill out
bpf_stack_build_id.ip fields (all other can be left uninitialized), and
update in place as we do build ID resolution.

We make sure to READ_ONCE() and cache locally current IP value as we
used it in a few places to find matching VMA and so on. Given this data
is directly accessible and modifiable by user's BPF code, we should make
sure to have a consistent view of it.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/stackmap.c | 49 +++++++++++++++++++++++++++++--------------
1 file changed, 33 insertions(+), 16 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 770ae8e88016..6457222b0b46 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -124,8 +124,18 @@ static struct bpf_map *stack_map_alloc(union bpf_attr *attr)
return ERR_PTR(err);
}

+/*
+ * Expects all id_offs[i].ip values to be set to correct initial IPs.
+ * They will be subsequently:
+ * - either adjusted in place to a file offset, if build ID fetching
+ * succeeds; in this case id_offs[i].build_id is set to correct build ID,
+ * and id_offs[i].status is set to BPF_STACK_BUILD_ID_VALID;
+ * - or IP will be kept intact, if build ID fetching failed; in this case
+ * id_offs[i].build_id is zeroed out and id_offs[i].status is set to
+ * BPF_STACK_BUILD_ID_IP.
+ */
static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
- u64 *ips, u32 trace_nr, bool user)
+ u32 trace_nr, bool user)
{
int i;
struct mmap_unlock_irq_work *work = NULL;
@@ -142,30 +152,28 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
/* cannot access current->mm, fall back to ips */
for (i = 0; i < trace_nr; i++) {
id_offs[i].status = BPF_STACK_BUILD_ID_IP;
- id_offs[i].ip = ips[i];
memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
}
return;
}

for (i = 0; i < trace_nr; i++) {
- if (range_in_vma(prev_vma, ips[i], ips[i])) {
+ u64 ip = READ_ONCE(id_offs[i].ip);
+
+ if (range_in_vma(prev_vma, ip, ip)) {
vma = prev_vma;
- memcpy(id_offs[i].build_id, prev_build_id,
- BUILD_ID_SIZE_MAX);
+ memcpy(id_offs[i].build_id, prev_build_id, BUILD_ID_SIZE_MAX);
goto build_id_valid;
}
- vma = find_vma(current->mm, ips[i]);
+ vma = find_vma(current->mm, ip);
if (!vma || build_id_parse_nofault(vma, id_offs[i].build_id, NULL)) {
/* per entry fall back to ips */
id_offs[i].status = BPF_STACK_BUILD_ID_IP;
- id_offs[i].ip = ips[i];
memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
continue;
}
build_id_valid:
- id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ips[i]
- - vma->vm_start;
+ id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ip - vma->vm_start;
id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
prev_vma = vma;
prev_build_id = id_offs[i].build_id;
@@ -216,7 +224,7 @@ 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 skip = flags & BPF_F_SKIP_FIELD_MASK;
- u32 hash, id, trace_nr, trace_len;
+ u32 hash, id, trace_nr, trace_len, i;
bool user = flags & BPF_F_USER_STACK;
u64 *ips;
bool hash_matches;
@@ -238,15 +246,18 @@ static long __bpf_get_stackid(struct bpf_map *map,
return id;

if (stack_map_use_build_id(map)) {
+ struct bpf_stack_build_id *id_offs;
+
/* for build_id+offset, pop a bucket before slow cmp */
new_bucket = (struct stack_map_bucket *)
pcpu_freelist_pop(&smap->freelist);
if (unlikely(!new_bucket))
return -ENOMEM;
new_bucket->nr = trace_nr;
- stack_map_get_build_id_offset(
- (struct bpf_stack_build_id *)new_bucket->data,
- ips, trace_nr, user);
+ id_offs = (struct bpf_stack_build_id *)new_bucket->data;
+ for (i = 0; i < trace_nr; i++)
+ id_offs[i].ip = ips[i];
+ stack_map_get_build_id_offset(id_offs, trace_nr, user);
trace_len = trace_nr * sizeof(struct bpf_stack_build_id);
if (hash_matches && bucket->nr == trace_nr &&
memcmp(bucket->data, new_bucket->data, trace_len) == 0) {
@@ -445,10 +456,16 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
copy_len = trace_nr * elem_size;

ips = trace->ip + skip;
- if (user && user_build_id)
- stack_map_get_build_id_offset(buf, ips, trace_nr, user);
- else
+ if (user && user_build_id) {
+ struct bpf_stack_build_id *id_offs = buf;
+ u32 i;
+
+ for (i = 0; i < trace_nr; i++)
+ id_offs[i].ip = ips[i];
+ stack_map_get_build_id_offset(buf, trace_nr, user);
+ } else {
memcpy(buf, ips, copy_len);
+ }

if (size > copy_len)
memset(buf + copy_len, 0, size - copy_len);
--
2.43.5