[Bug middle-end/79257] spurious -Wformat-overflow=1 warning with -O2 and sanitizer

[catwell-gcc at catwell dot info]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79257

Pierre Chapuis  changed:

   What|Removed |Added

 CC||catwell-gcc at catwell dot info

--- Comment #5 from Pierre Chapuis  ---
I think I can reproduce something similar *without* the sanitizer.

Using GCC 7.1.1, with:


#include 
int main () {
int i; char obuf[3];
int start = 0x00;
for (i = start; i <= 0xff; ++i) {
sprintf(obuf, "%02x", i);
}
return 0;
}


I get:


demo.c: In function ‘main’:
demo.c:6:23: warning: ‘sprintf’ may write a terminating nul past the end of the
destination [-Wformat-overflow=]
 sprintf(obuf, "%02x", i);
   ^~
demo.c:6:9: note: ‘sprintf’ output between 3 and 4 bytes into a destination of
size 3
 sprintf(obuf, "%02x", i);
 ^~~~


At O1 I don't get the warning.

If I set `start` to `0xfb` or `0xfc`, I get the same warning. If I set it to
`0xfe` or `0xff` I don't get a warning. If I set it to `0xfd` I get the warning
*twice*:


demo.c: In function ‘main’:
demo.c:6:23: warning: ‘sprintf’ may write a terminating nul past the end of the
destination [-Wformat-overflow=]
 sprintf(obuf, "%02x", i);
   ^~
demo.c:6:9: note: ‘sprintf’ output between 3 and 4 bytes into a destination of
size 3
 sprintf(obuf, "%02x", i);
 ^~~~
demo.c:6:23: warning: ‘sprintf’ may write a terminating nul past the end of the
destination [-Wformat-overflow=]
 sprintf(obuf, "%02x", i);
   ^~
demo.c:6:9: note: ‘sprintf’ output between 3 and 4 bytes into a destination of
size 3
 sprintf(obuf, "%02x", i);
 ^~~~

[Bug middle-end/79257] spurious -Wformat-overflow=1 warning with -O2 and sanitizer

[catwell-gcc at catwell dot info]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79257

--- Comment #6 from Pierre Chapuis  ---
(Note this is my first time reporting something to GCC, if you think I should
have opened a new ticked because it may be a slightly different bug please tell
me.)

[Bug c/81401] New: False positive sprintf warning at O2 (-Wformat-overflow)

[catwell-gcc at catwell dot info]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81401

Bug ID: 81401
   Summary: False positive sprintf warning at O2
(-Wformat-overflow)
   Product: gcc
   Version: 7.1.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: catwell-gcc at catwell dot info
  Target Milestone: ---

Using GCC 7.1.1, with:


#include 
int main () {
int i; char obuf[3];
int start = 0x00;
for (i = start; i <= 0xff; ++i) {
sprintf(obuf, "%02x", i);
}
return 0;
}


I get:


demo.c: In function ‘main’:
demo.c:6:23: warning: ‘sprintf’ may write a terminating nul past the end of the
destination [-Wformat-overflow=]
 sprintf(obuf, "%02x", i);
   ^~
demo.c:6:9: note: ‘sprintf’ output between 3 and 4 bytes into a destination of
size 3
 sprintf(obuf, "%02x", i);
 ^~~~


At O1 I don't get the warning.

If I set `start` to `0xfb` or `0xfc`, I get the same warning. If I set it to
`0xfe` or `0xff` I don't get a warning. If I set it to `0xfd` I get the warning
*twice*:


demo.c: In function ‘main’:
demo.c:6:23: warning: ‘sprintf’ may write a terminating nul past the end of the
destination [-Wformat-overflow=]
 sprintf(obuf, "%02x", i);
   ^~
demo.c:6:9: note: ‘sprintf’ output between 3 and 4 bytes into a destination of
size 3
 sprintf(obuf, "%02x", i);
 ^~~~
demo.c:6:23: warning: ‘sprintf’ may write a terminating nul past the end of the
destination [-Wformat-overflow=]
 sprintf(obuf, "%02x", i);
   ^~
demo.c:6:9: note: ‘sprintf’ output between 3 and 4 bytes into a destination of
size 3
 sprintf(obuf, "%02x", i);
 ^~~~

I was advised to open a new bug for this at [1] even though it looks similar to
[2].

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79257#c8
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78969#c4