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

[Sergio Durigan Junior <sergiodj at redhat dot com>]

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

-- 
Sergio
GPG key ID: 237A 54B1 0287 28BF 00EF  31F4 D0EB 7628 65FC 5E36
Please send encrypted e-mail if possible
http://sergiodj.net/