GDB: function parameters have garbage values on function entry

[Alex Coplan]

After upgrading from Ubuntu 18.04 to Ubuntu 20.04 on my x86_64 desktop machine, I've noticed a significant regression in the debug experience with gdb. In particular, function parameters in GDB now seem to reliably have garbage values on entry to the function until I step once inside the function. Here is a reproducer:

$ cat test.c
#include <stdio.h>
int f(int x, int y) {
  printf ("%d, %d\n", x, y);
}
int main(void) {
  f(2,3);
}
$ gcc -g3 test.c
$ gdb a.out
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
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<http://www.gnu.org/software/gdb/bugs/>.
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Reading symbols from a.out...
(gdb) b f
Breakpoint 1 at 0x1149: file test.c, line 2.
(gdb) r
Starting program: /data_sdb/toolchain/a.out

Breakpoint 1, f (x=21845, y=1431654496) at test.c:2
2 int f(int x, int y) {
(gdb) p x
$1 = 21845
(gdb) p y
$2 = 1431654496
(gdb) n
3 printf ("%d, %d\n", x, y);
(gdb) p x
$3 = 2
(gdb) p y
$4 = 3

I tried compiling GDB from source and noticed that I could reproduce the problem with GDB 9 but not with GDB 10. A bisection showed that the following GDB commit fixed the issue: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ac4a4f1cd7dceeeb17d0b8c077c874f2247acbf0

Perhaps that patch should be backported to GDB 9, but it's not entirely clear why the prologue analysis is even necessary in this case.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gdb 9.2-0ubuntu1~20.04.1
ProcVersionSignature: Ubuntu 5.4.0-139.156-generic 5.4.224
Uname: Linux 5.4.0-139-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.25
Architecture: amd64
CasperMD5CheckResult: skip
Date: Tue Mar 7 16:01:56 2023
SourcePackage: gdb
UpgradeStatus: Upgraded to focal on 2023-03-03 (4 days ago)