mcount: Fix lost diff event from read trigger with argument tracing#2048
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After apply this root@server:~/original/uftrace# uftrace -a -T a@read=proc/statm -F main tests/t-abc
# DURATION TID FUNCTION
[3137706] | main(1, 0x7fff47757a28) {
[3137706] | a() {
[3137706] | /* read:proc/statm (vmsize=13316KB vmrss=7344KB s>
[3137706] | b() {
[3137706] | c() {
0.327 us [3137706] | getpid() = 3137706;
3.936 us [3137706] | } = 37706; /* c */
4.403 us [3137706] | } = 37707; /* b */
[3137706] | /* diff:proc/statm (vmsize=+0KB vmrss=+4KB shared>
24.099 us [3137706] | } = 37706; /* a */
31.528 us [3137706] | } = 0; /* main */
root@server:~/original/uftrace# uftrace -T a@read=proc/statm -F main tests/t-abc
# DURATION TID FUNCTION
[3137743] | main() {
[3137743] | a() {
[3137743] | /* read:proc/statm (vmsize=13068KB vmrss=6888KB s>
[3137743] | b() {
[3137743] | c() {
0.324 us [3137743] | getpid();
1.297 us [3137743] | } /* c */
1.634 us [3137743] | } /* b */
[3137743] | /* diff:proc/statm (vmsize=+0KB vmrss=+0KB shared>
22.662 us [3137743] | } /* a */
28.724 us [3137743] | } /* main */diff is normally working. |
namhyung
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May 23, 2026
namhyung
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LGTM, thanks for fixing this.
Nit: the subject prefix would be "mcount" instead of "libmcount".
When `-A`, `-R`, or `-a` is combined with a function-level `@read`
trigger, the `diff` event at function exit is lost:
$ uftrace -A 'a@arg1/u64' -T a@read=proc/statm -F main tests/t-abc
# DURATION TID FUNCTION
[3162404] | main() {
[3162404] | a(1) {
[3162404] | /* read:proc/statm (vmsize=13192KB vmrss=7192KB ...) */
[3162404] | b() {
[3162404] | c() {
0.303 us [3162404] | getpid();
1.602 us [3162404] | } /* c */
1.902 us [3162404] | } /* b */
19.249 us [3162404] | } /* a */ <-- diff:proc/statm is missing
19.793 us [3162404] | } /* main */
In save_trigger_read(), arg_data is meant to point to the end of
the argument region so that an event written at the buffer tail does
not overwrite it. The argument size lives at the start of argbuf as
a uint32_t, but the code reads it from ptr (= argbuf + event_idx),
which on the exit-time call points to the read event header saved at
entry. Its first 4 bytes are the lower 32 bits of event->time, so
arg_data jumps far outside the buffer and the overwrite check
rejects the new diff event.
Fix it by reading the size from the start of the argbuf, and adding
the size header itself when computing the end of the region.
Signed-off-by: Jun Yeong Kim <junyeonggim5@gmail.com>
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Thanks for the review. I just updated the prefix to mcount. |
Owner
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Thanks, please do the same in the callsite PR. |
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When
-A,-R, or-ais combined with a function-level@readtrigger, thediffevent at function exit is lost:In save_trigger_read(), arg_data is meant to point to the end of the argument region so that an event written at the buffer tail does not overwrite it. The argument size lives at the start of argbuf as a uint32_t, but the code reads it from ptr (= argbuf + event_idx), which on the exit-time call points to the read event header saved at entry. Its first 4 bytes are the lower 32 bits of event->time, so arg_data jumps far outside the buffer and the overwrite check rejects the new diff event.
Fix it by reading the size from the start of the argbuf, and adding the size header itself when computing the end of the region.