Re: [PATCH] Fix printf of a convenience variable holding an inferior address

[Sergio Durigan Junior <sergiodj@redhat.com>]

On Tuesday, March 03 2020, I wrote:

> On Tuesday, March 03 2020, Andrew Burgess wrote:
>
>> * Sergio Durigan Junior <sergiodj@redhat.com> [2020-03-01 21:46:16 -0500]:
>>
>>> Back at:
>>> 
>>> commit 1f6f6e21fa86dc3411a6498608f32e9eb24b7851
>>> Author: Philippe Waroquiers <philippe.waroquiers@skynet.be>
>>> Date:   Mon Jun 10 21:41:51 2019 +0200
>>> 
>>>     Ensure GDB printf command can print convenience var strings without a target.
>>> 
>>> GDB was extended in order to allow the printing of convenience
>>> variables that are strings without a target.  However, this introduced
>>> a regression that hasn't been caught by our testsuite (because there
>>> were no tests for it).
>>> 
>>> The problem happens when we try to print a convenience variable that
>>> holds the address of a string in the inferior.  The following
>>> two-liners can reproduce the issue:
>>> 
>>> $ echo -e 'int main(){const char a[]="test";return 0;}' | gcc -x c - -O0-g3
>>> $ ./gdb/gdb --data-directory ./gdb/data-directory -q ./a.out -ex 'start' -ex 'set $x = (const char *) (&a[0] + 2)' -ex 'printf "%s\n", $x'
>>> 
>>> After some investigation, I found that the problem happens on
>>> printcmd.c:printf_c_string.  In the case above, we're taking the first
>>> branch of the 'if' condition, which assumes that there will be a value
>>> to be printed at "value_contents (value)".  There isn't.  We actually
>>> need to obtain the address that the variable points to, and read the
>>> contents from memory.
>>> 
>>> It seems to me that we should avoid this branch if the TYPE_CODE of
>>> "value_type (value)" is TYPE_CODE_PTR (i.e., a pointer to the
>>> inferior's memory).  This is what this patch does.
>>> 
>>> I took the liberty to extend the current testcase under
>>> gdb.base/printcmds.exp and create a test that exercises this scenario.
>>> 
>>> No regressions have been found on Buildbot.
>>> 
>>> gdb/ChangeLog:
>>> 2020-03-02  Sergio Durigan Junior  <sergiodj@redhat.com>
>>> 
>>> 	* printcmd.c (print_c_string): Check also for TYPE_CODE_PTR
>>> 	when verifying if dealing with a convenience variable.
>>> 
>>> gdb/testsuite/ChangeLog:
>>> 2020-03-02  Sergio Durigan Junior  <sergiodj@redhat.com>
>>> 
>>> 	* gdb.base/printcmds.exp: Add test to verify printf of a
>>> 	variable holding an address.
>>
>> LGTM.
>
> Thanks, pushed:
>
> 7b973adce2b486518d3150db257b179e1b9a5d33

BTW, this also affects 9.1 (in fact, that's where I found the bug).  I
can create a tracking bug and backport it to the branch if the issue and
fix are deemed important enough.

Thanks,

-- 
Sergio
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