[binutils-gdb] [gdb/testsuite] Make gdb.base/longjmp.exp FAIL more stable across archs

[Tom de Vries <vries@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=6e41445bb006f3afc784862f8eb1bf0f2691a94a

commit 6e41445bb006f3afc784862f8eb1bf0f2691a94a
Author: Tom de Vries <tdevries@suse.de>
Date:   Tue Dec 6 12:01:46 2022 +0100

    [gdb/testsuite] Make gdb.base/longjmp.exp FAIL more stable across archs
    
    When running test-case gdb.base/longjmp.exp on x86_64-linux, the master
    longjmp breakpoint is set using probes and the test-case passes:
    ...
    (gdb) PASS: gdb.base/longjmp.exp: next to longjmp (1)
    next^M
    0x00000000004005cc      49        if (setjmp (env) == 0) /* patt1 */^M
    (gdb) PASS: gdb.base/longjmp.exp: next over longjmp(1)
    next^M
    56            resumes++;^M
    (gdb) PASS: gdb.base/longjmp.exp: next into else block (1)
    ...
    
    However, if I disable
    create_longjmp_master_breakpoint_probe, we have instead:
    ...
    (gdb) PASS: gdb.base/longjmp.exp: next to longjmp (1)
    next^M
    56            resumes++;^M
    (gdb) FAIL: gdb.base/longjmp.exp: next over longjmp(1)
    ...
    
    At first glance, the failure mode doesn't look too bad: we stop
    a few insns later than the passing scenario.
    
    For contrast, if we do the same on powerpc64le, the failure mode is:
    ...
    (gdb) PASS: gdb.base/longjmp.exp: next to longjmp (1)
    next^M
    ^M
    Breakpoint 3, main () at longjmp.c:59^M
    59        i = 1; /* miss_step_1 */^M
    (gdb) FAIL: gdb.base/longjmp.exp: next over longjmp(1)
    ...
    Here we only stop because of running into the safety net breakpoint at
    miss_step_1.
    
    So, how does this happen on x86_64?  Let's look at the code:
    ...
    4005c7: e8 94 fe ff ff    call 400460 <_setjmp@plt>
    4005cc: 85 c0             test %eax,%eax
    4005ce: 75 1e             jne  4005ee <main+0x3b>
    4005d0: 8b 05 8e 0a 20 00 mov  0x200a8e(%rip),%eax # 601064 <longjmps>
    4005d6: 83 c0 01          add  $0x1,%eax
    4005d9: 89 05 85 0a 20 00 mov  %eax,0x200a85(%rip) # 601064 <longjmps>
    4005df: be 01 00 00 00    mov  $0x1,%esi
    4005e4: bf 80 10 60 00    mov  $0x601080,%edi
    4005e9: e8 82 fe ff ff    call 400470 <longjmp@plt>
    4005ee: 8b 05 74 0a 20 00 mov  0x200a74(%rip),%eax # 601068 <resumes>
    ...
    The next over the longjmp call at 4005e9 is supposed to stop at the longjmp
    target at 4005cc, but instead we stop at 4005ee, where we have the step-resume
    breakpoint inserted by the next.  In other words, we accidentally "return"
    from the longjmp call to the insn immediately after it (even though
    a longjmp is a noreturn function).
    
    Try to avoid this accident and make the failure mode on x86_64 the same as on
    powerpc64le, by switching the then and else branch.
    
    Tested on x86_64-linux.

Diff:
---
 gdb/testsuite/gdb.base/longjmp.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/gdb/testsuite/gdb.base/longjmp.c b/gdb/testsuite/gdb.base/longjmp.c
index 4139e49e6f1..ce6990ca99a 100644
--- a/gdb/testsuite/gdb.base/longjmp.c
+++ b/gdb/testsuite/gdb.base/longjmp.c
@@ -46,14 +46,14 @@ main ()
   volatile int i = 0;
 
   /* Pattern 1 - simple longjmp.  */
-  if (setjmp (env) == 0) /* patt1 */
+  if (setjmp (env) != 0) /* patt1 */
     {
-      longjmps++;
-      longjmp (env, 1);
+      resumes++;
     }
   else
     {
-      resumes++;
+      longjmps++;
+      longjmp (env, 1);
     }
 
   i = 1; /* miss_step_1 */