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fix futexctn tool for symbol resolution and infinite loop #5496

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fix futexctn tool for symbol resolution and infinite loop #5496

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964e561
bcc/libbpf-tools: fix futexctn stack trace symbol resolution
Mar 10, 2026
54164f5
bcc/libbpf-tools: fix futexctn infinite loop in hists map cleanup
Mar 20, 2026
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54 changes: 51 additions & 3 deletions libbpf-tools/futexctn.c
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Original file line number Diff line number Diff line change
Expand Up @@ -232,6 +232,11 @@ static int print_stack(struct futexctn_bpf *obj, struct hist_key *info)
printf(" %s\n", sym->name);
}
#else
/*
* sym expected to be cached in preload stage
* too late to cache here for short lived processes
* proc/pid/maps not available to cache
*/

@ekyooo ekyooo Aug 10, 2026

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Could we address this using blazesym's blaze_symbolize_cache_process() instead of changing the existing symbol cache infrastructure?

syms = syms_cache__get_syms(syms_cache, info->pid_tgid >> 32);
if (!syms) {
if (!env.verbose) {
Expand Down Expand Up @@ -278,6 +283,9 @@ static int print_map(struct futexctn_bpf *obj)
const char *units = env.milliseconds ? "msecs" : "usecs";
int err,fd = bpf_map__fd(obj->maps.hists);
struct hist hist;
struct hist_key *ss_keys = NULL;
int ss_key_cnt = 0;
int max_entries = 0;

while (!bpf_map_get_next_key(fd, &lookup_key, &next_key)) {
err = bpf_map_lookup_elem(fd, &next_key, &hist);
Expand All @@ -301,16 +309,33 @@ static int print_map(struct futexctn_bpf *obj)
lookup_key = next_key;
}

/* snapshot all existing keys */
max_entries = bpf_map__max_entries(obj->maps.hists);
ss_keys = malloc(max_entries * sizeof(*ss_keys));

if (!ss_keys) {
fprintf(stderr, "failed to alloc memory for hist cleanup\n");
return -1;
}

lookup_key.pid_tgid = -1;
while (!bpf_map_get_next_key(fd, &lookup_key, &next_key)) {
err = bpf_map_delete_elem(fd, &next_key);
while (!bpf_map_get_next_key(fd, &lookup_key, &next_key) &&

@ekyooo ekyooo Aug 10, 2026

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Could we use dump_hash() with lookup_and_delete = true here? See biotop.c for an example.

ss_key_cnt < max_entries) {
ss_keys[ss_key_cnt++] = next_key;
lookup_key = next_key;
}

/* delete snapshotted keys explicitly */
for (int i = 0; i < ss_key_cnt; i++) {
err = bpf_map_delete_elem(fd, &ss_keys[i]);
if (err < 0) {
fprintf(stderr, "failed to cleanup hist : %d\n", err);
free(ss_keys);
return -1;
}
lookup_key = next_key;
}

free(ss_keys);
return 0;
}

Expand Down Expand Up @@ -375,7 +400,30 @@ int main(int argc, char **argv)

/* main: poll */
while (1) {

#ifndef USE_BLAZESYM
for (int elapsed = 0; elapsed < env.interval; elapsed++) {
sleep(1);
if (!env.summary) {
/*
* Populate symbol cache periodically during the interval
* while processes are still alive and /proc/<pid>/maps
* is readable. A single preload before or after
* sleep(interval) misses very short-lived processes that
* both start and exit within the sleep window of 1s.
*/
struct hist_key preload_key = { .pid_tgid = -1 }, next_key;
int preload_fd = bpf_map__fd(obj->maps.hists);

while (!bpf_map_get_next_key(preload_fd, &preload_key, &next_key)) {
syms_cache__get_syms(syms_cache, next_key.pid_tgid >> 32);
preload_key = next_key;
}
}
}
#else
sleep(env.interval);
#endif
printf("\n");

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dubeyabhishek marked this conversation as resolved.
if (env.timestamp) {
Expand Down
13 changes: 12 additions & 1 deletion libbpf-tools/trace_helpers.c
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Original file line number Diff line number Diff line change
Expand Up @@ -800,10 +800,21 @@ struct syms *syms_cache__get_syms(struct syms_cache *syms_cache, int tgid)
{
void *tmp;
int i;
struct syms *syms = NULL;

for (i = 0; i < syms_cache->nr; i++) {
if (syms_cache->data[i].tgid == tgid)
if (syms_cache->data[i].tgid == tgid) {
/*
* for caching symbols appearing after last
* pre-loading/caching cycle.
*/
if (!syms_cache->data[i].syms) {
syms = syms__load_pid(tgid);
if (syms)
syms_cache->data[i].syms = syms;
}
return syms_cache->data[i].syms;
}
}

tmp = realloc(syms_cache->data, (syms_cache->nr + 1) *
Expand Down
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