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

[Andrew Burgess <andrew.burgess@embecosm.com>]

* 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,
Andrew


> ---
>  gdb/printcmd.c                       | 3 ++-
>  gdb/testsuite/gdb.base/printcmds.exp | 8 ++++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/printcmd.c b/gdb/printcmd.c
> index 797041484e..78d8d3d81e 100644
> --- a/gdb/printcmd.c
> +++ b/gdb/printcmd.c
> @@ -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));
> diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
> index bd2afc8696..c87a1517f0 100644
> --- a/gdb/testsuite/gdb.base/printcmds.exp
> +++ b/gdb/testsuite/gdb.base/printcmds.exp
> @@ -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
> -- 
> 2.24.1
>