[RFA,3/3] Special case NULL when using printf's %s format

Message ID 20180215205001.337-4-tom@tromey.com
State New
Headers show
Series
  • a few minor printf fixes
Related show

Commit Message

Tom Tromey Feb. 15, 2018, 8:50 p.m.
This changes the printf command's %s and %ls formats to special-case
NULL, and print "(null)" for these.  This is PR cli/14977.  This
behavior seems a bit friendlier; I was undecided on whether other
invalid pointers should be handled specially somehow, so for the time
being I've left those out.

gdb/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	PR cli/14977:
	* printcmd.c (printf_c_string, printf_wide_c_string): Special case
	for NULL.

gdb/gdbserver/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	PR cli/14977:
	* ax.c (ax_printf): Special case for NULL.

gdb/testsuite/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	PR cli/14977:
	* gdb.base/printcmds.exp (test_printf): Add printf test of %s with
	a null pointer.
	* gdb.base/wchar.exp: Likewise.
---
 gdb/ChangeLog                        |  6 ++++++
 gdb/gdbserver/ChangeLog              |  5 +++++
 gdb/gdbserver/ax.c                   |  5 +++++
 gdb/printcmd.c                       | 10 ++++++++++
 gdb/testsuite/ChangeLog              |  7 +++++++
 gdb/testsuite/gdb.base/printcmds.exp |  3 +++
 gdb/testsuite/gdb.base/wchar.exp     |  3 +++
 7 files changed, 39 insertions(+)

-- 
2.13.6

Comments

Pedro Alves Feb. 16, 2018, 12:08 p.m. | #1
On 02/15/2018 08:50 PM, Tom Tromey wrote:
> This changes the printf command's %s and %ls formats to special-case

> NULL, and print "(null)" for these.  This is PR cli/14977.  This

> behavior seems a bit friendlier; I was undecided on whether other

> invalid pointers should be handled specially somehow, so for the time

> being I've left those out.


A question here is what to do on targets that actually map things at
address zero.  IIRC, some ARM chips do that.  For such ports, I think you'd
want to be able to read/print that memory.  I thought we had a gdbarch hook
for that, but I'm not finding it right now.  Maybe I was thinking of
has_section_at_zero in dwarf2read.c.

Thanks,
Pedro Alves
Pedro Alves Feb. 16, 2018, 12:25 p.m. | #2
On 02/16/2018 12:08 PM, Pedro Alves wrote:
> On 02/15/2018 08:50 PM, Tom Tromey wrote:

>> This changes the printf command's %s and %ls formats to special-case

>> NULL, and print "(null)" for these.  This is PR cli/14977.  This

>> behavior seems a bit friendlier; I was undecided on whether other

>> invalid pointers should be handled specially somehow, so for the time

>> being I've left those out.

> 

> A question here is what to do on targets that actually map things at

> address zero.  IIRC, some ARM chips do that.  For such ports, I think you'd

> want to be able to read/print that memory.  I thought we had a gdbarch hook

> for that, but I'm not finding it right now.  Maybe I was thinking of

> has_section_at_zero in dwarf2read.c.


Then again, I don't think it's likely that programs map
strings at 0 (I think it's more like interrupt vectors), so for %s,
it's likely that nobody would miss printing 0 as a string.

So with that in mind, this is fine with me.

Thanks,
Pedro Alves
Tom Tromey Feb. 16, 2018, 9:58 p.m. | #3
Pedro> Then again, I don't think it's likely that programs map
Pedro> strings at 0 (I think it's more like interrupt vectors), so for %s,
Pedro> it's likely that nobody would miss printing 0 as a string.

Another option would be to do a try-catch and only print "(null)" on
failure, if the pointer is 0.

Tom

Patch

diff --git a/gdb/gdbserver/ax.c b/gdb/gdbserver/ax.c
index b42ee54c84..c754383df8 100644
--- a/gdb/gdbserver/ax.c
+++ b/gdb/gdbserver/ax.c
@@ -847,6 +847,11 @@  ax_printf (CORE_ADDR fn, CORE_ADDR chan, const char *format,
 	    int j;
 
 	    tem = args[i];
+	    if (tem == 0)
+	      {
+		printf (current_substring, "(null)");
+		break;
+	      }
 
 	    /* This is a %s argument.  Find the length of the string.  */
 	    for (j = 0;; j++)
diff --git a/gdb/printcmd.c b/gdb/printcmd.c
index d2d13895f6..a399ed761b 100644
--- a/gdb/printcmd.c
+++ b/gdb/printcmd.c
@@ -2220,6 +2220,11 @@  printf_c_string (struct ui_file *stream, const char *format,
   int j;
 
   tem = value_as_address (value);
+  if (tem == 0)
+    {
+      fprintf_filtered (stream, format, "(null)");
+      return;
+    }
 
   /* This is a %s argument.  Find the length of the string.  */
   for (j = 0;; j++)
@@ -2260,6 +2265,11 @@  printf_wide_c_string (struct ui_file *stream, const char *format,
   gdb_byte *buf = (gdb_byte *) alloca (wcwidth);
 
   tem = value_as_address (value);
+  if (tem == 0)
+    {
+      fprintf_filtered (stream, format, "(null)");
+      return;
+    }
 
   /* This is a %s argument.  Find the length of the string.  */
   for (j = 0;; j += wcwidth)
diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
index 56cedb908f..635bd621fd 100644
--- a/gdb/testsuite/gdb.base/printcmds.exp
+++ b/gdb/testsuite/gdb.base/printcmds.exp
@@ -780,6 +780,9 @@  proc test_printf {} {
     # PR cli/19918.
     gdb_test "printf \"%-16dq\\n\", 0" "0               q"
     gdb_test "printf \"%-16pq\\n\", 0" "\\(nil\\)           q"
+
+    # PR cli/14977.
+    gdb_test "printf \"%s\\n\", 0" "\\(null\\)"
 }
 
 #Test printing DFP values with printf
diff --git a/gdb/testsuite/gdb.base/wchar.exp b/gdb/testsuite/gdb.base/wchar.exp
index 80b6513e22..a4d9bce7d0 100644
--- a/gdb/testsuite/gdb.base/wchar.exp
+++ b/gdb/testsuite/gdb.base/wchar.exp
@@ -69,3 +69,6 @@  gdb_test "print repeat" "= L\"A$cent$cent\"\.\.\." \
 
 gdb_test "print repeat_p" "= $hex L\"A$cent$cent\"\.\.\." \
     "print repeat_p (print elements 3)"
+
+# From PR cli/14977, but here because it requires wchar_t.
+gdb_test "printf \"%ls\\n\", 0" "\\(null\\)"