Bug 31064 - [gdb/testsuite] FAIL: gdb.base/examine-backward.exp: char-width=1, print-max=20: take 6 strings backward (pattern 1)
Summary: [gdb/testsuite] FAIL: gdb.base/examine-backward.exp: char-width=1, print-max=...
Status: RESOLVED FIXED
Alias: None
Product: gdb
Classification: Unclassified
Component: testsuite (show other bugs)
Version: HEAD
: P2 normal
Target Milestone: 15.1
Assignee: Not yet assigned to anyone
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Depends on:
Blocks:
 
Reported: 2023-11-13 15:05 UTC by Tom de Vries
Modified: 2023-11-21 12:14 UTC (History)
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Description Tom de Vries 2023-11-13 15:05:54 UTC 
On a pinebook I run into:
...
(gdb) PASS: gdb.base/examine-backward.exp: char-width=1, print-max=20: take 1 char backward
gdb_expect_list pattern: /"ABCDEFGHIJKLMNOPQRST".../
x/-6s^M
0xaaabb03c:     "<\260\253\252ABCDEFGHIJKLMNOP"...^M
0xaaabb050 <TestStrings+16>:    "QRSTUVWXYZ"^M
0xaaabb05b <TestStrings+27>:    ""^M
0xaaabb05c <TestStrings+28>:    ""^M
0xaaabb05d <TestStrings+29>:    "\343\201\273\343\201\222\343\201\273\343\201\222"^M
0xaaabb06a <TestStrings+42>:    "01234567890123456789"...^M
(gdb) FAIL: gdb.base/examine-backward.exp: char-width=1, print-max=20: take 6 strings backward (pattern 1)
...

It's clear that this:
...
/* This is here just to ensure we have a null character before                                     
   TestStrings, to avoid showing garbage when we look for strings                                  
   backwards from TestStrings.  */

unsigned char Barrier[] = {
  0x00,
};
...
is not working:
...
$ nm outputs/gdb.base/examine-backward/examine-backward | egrep "TestStrings|Barrier"
00011250 B Barrier
00011040 D TestStrings
00011094 D TestStringsH
00011128 D TestStringsW
...
Comment 2 Sourceware Commits 2023-11-21 12:13:36 UTC 
The master branch has been updated by Tom de Vries <vries@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=42ffc15774fc791f2ac9a719e81589c8e91bdb98

commit 42ffc15774fc791f2ac9a719e81589c8e91bdb98
Author: Tom de Vries <tdevries@suse.de>
Date:   Tue Nov 21 13:15:29 2023 +0100

    [gdb/testsuite] Fix spurious FAILs with examine-backward.exp, again
    
    Commit 59a561480d5 ("Fix spurious FAILs with examine-backward.exp") describes
    the problem that:
    ...
    The test case examine-backward.exp issues the command "x/-s" after the end
    of the first string in TestStrings, but without making sure that this
    string is preceded by a string terminator.  Thus GDB may spuriously print
    some random characters from before that string, and then the test fails.
    ...
    
    The commit fixes the problem by adding a Barrier variable before the TestStrings
    variable:
    ...
    +const char Barrier[] = { 0x0 };
     const char TestStrings[] = {
    ...
    
    There is however no guarantee that Barrier is placed immediately before
    TestStrings.
    
    Before recent commit 169fe7ab54b ("Change gdb.base/examine-backwards.exp for
    AIX.") on x86_64-linux, I see:
    ...
    0000000000400660 R Barrier
    0000000000400680 R TestStrings
    ...
    
    So while the Barrier variable is the first before the TestStrings variable,
    it's not immediately preceding TestStrings.
    
    After commit 169fe7ab54b:
    ...
    0000000000402259 B Barrier
    0000000000402020 D TestStrings
    ...
    they're not even in the same section anymore.
    
    Fix this reliably by adding the zero in the array itself:
    ...
    char TestStringsBase[] = {
      0x0,
      ...
    };
    char *TestStrings = &TestStringsBase[1];
    ...
    and do likewise for TestStringsH and TestStringsW.
    
    Tested on x86_64-linux.
    
    PR testsuite/31064
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31064
Comment 3 Tom de Vries 2023-11-21 12:14:57 UTC 
Fixed.