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Fix printf of a convenience variable holding an inferior address

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commit 1f6f6e21fa
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:
2020-03-03  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:
2020-03-03  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/printcmds.exp: Add test to verify printf of a
	variable holding an address.
This commit is contained in:
Sergio Durigan Junior 2020-03-01 18:15:53 -05:00
parent 5be2b2f549
commit 7b973adce2
4 changed files with 20 additions and 1 deletions
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5
gdb/ChangeLog
View file

@ -1,3 +1,8 @@
2020-03-03 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.
2020-03-03 Luis Machado <luis.machado@linaro.org>
* auxv.c (default_print_auxv_entry): Add new AUXV entries.

3
gdb/printcmd.c
View file

@ -2260,7 +2260,8 @@ printf_c_string (struct ui_file *stream, const char *format,
{
const gdb_byte *str;
if (VALUE_LVAL (value) == lval_internalvar
if (TYPE_CODE (value_type (value)) != TYPE_CODE_PTR
&& VALUE_LVAL (value) == lval_internalvar
&& c_is_string_type_p (value_type (value)))
{
size_t len = TYPE_LENGTH (value_type (value));

5
gdb/testsuite/ChangeLog
View file

@ -1,3 +1,8 @@
2020-03-03 Sergio Durigan Junior <sergiodj@redhat.com>
* gdb.base/printcmds.exp: Add test to verify printf of a
variable holding an address.
2020-03-03 Tom de Vries <tdevries@suse.de>
* gdb.mi/gdb2549.exp: Fix "register values t" check-read1 timeout.

8
gdb/testsuite/gdb.base/printcmds.exp
View file

@ -1039,6 +1039,14 @@ gdb_test_no_output "set may-call-functions off"
test_printf_convenience_var "with target, may-call-functions off"
gdb_test_no_output "set may-call-functions on"
# Test printf of a variable that holds the address to a substring in
# the inferior. This test will not work without a target.
gdb_test_no_output "set var \$test_substr = \(char \*\) \(&teststring\[0\] + 4\)" \
"set \$test_substr var"
gdb_test "printf \"test_substr val = %s\\n\", \$test_substr" \
"test_substr val = string contents" \
"print \$test_substr"
test_integer_literals_accepted
test_integer_literals_rejected
test_float_accepted