[Bug cli/14011] New: GDB uses strcpy() with undefined behaviour,causing bug in CLI cd_command().

["fredrik dot hederstierna at securitas-direct dot com"<sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14011

             Bug #: 14011
           Summary: GDB uses strcpy() with undefined behaviour, causing
                    bug in CLI cd_command().
           Product: gdb
           Version: 7.4
            Status: NEW
          Severity: normal
          Priority: P2
         Component: cli
        AssignedTo: unassigned@sourceware.org
        ReportedBy: fredrik.hederstierna@securitas-direct.com
    Classification: Unclassified


The C standard states that the behavior of strcpy() is undefined when the
source and destination objects overlap.
Undefined behavior means it may work sometimes, or it may fail, or it may
appear to succeed but manifest failure elsewhere in the program.

I got a failure running arm-elf-gdb-4.7.0 (compiled with GCC-4.6.1-9ubuntu3)
with arguments

  arm-elf-gdb --cd=../../build/sniffer2/ sniffer2.elf

...
Reading symbols from
/home/fredrikh/workspace/buile/sniffer2/sniffer2.elf...done.
(gdb)

Note that letter 'd' in 'build' is overwritten with letter 'e' in current_path.
The path to 'buile' is non-existing causing error.

I tracked down to the cd_command() function in CLI that was causing the bug.
It seems like the code is doing strcpy() on overlapping regions, to eliminate
".." paths, this causing an undefined behaviour.
GDB corrupted the dir-path replacing one letter:

The standard solution is to replace strcpy() with memmove(), and I submit a
proposed patch that fixed the bug.



Index: gdb/cli/cli-cmds.c
===================================================================
RCS file: /cvs/src/src/gdb/cli/cli-cmds.c,v
retrieving revision 1.128
diff -r1.128 cli-cmds.c
420c420
<       strcpy (p, p + 2);
---
>       memmove(p, p + 2, strlen(p + 2) + 1);
439c439
<                 strcpy (q - 1, p + 3);
---
>                 memmove(q - 1, p + 3, strlen(p + 3) + 1);



I fear though that there might be more cases in the sources where strcpy() is
used this way.
Maybe its a good idea to grep 'strcpy' and check that all uses are safe and
non-overlapping.

Another idea is to use a custom gdb_strcpy() instead, that we know always copy
from left-to-right, where we do define behaviour in the overlapping case.
Though is a danger to have dependencies on external C-lib implementation of
string functions.

Thanks & Best Regards,

Fredrik Hederstierna
Securitas Direct AB
Malmoe Sweden

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