[PATCH] Don't pass NULL to memcpy in gdb

[PATCH] Don't pass NULL to memcpy in gdb

[Tom Tromey]

I compiled gdb with -fsanitize=undefined and ran the test suite.

A couple of reports came from passing NULL to memcpy, e.g.:

[...]btrace-common.cc:176:13: runtime error: null pointer passed as argument 2, which is declared to never be null

While it would be better to fix this in the standard, in the meantime
it seems easy to avoid this error.

gdb/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* dwarf2/abbrev.c (abbrev_table::read): Conditionally call
	memcpy.

gdbsupport/ChangeLog
2020-03-20  Tom Tromey  <tromey@adacore.com>

	* btrace-common.cc (btrace_data_append): Conditionally call
	memcpy.
---
 gdb/ChangeLog               | 5 +++++
 gdb/dwarf2/abbrev.c         | 5 +++--
 gdbsupport/ChangeLog        | 5 +++++
 gdbsupport/btrace-common.cc | 3 ++-
 4 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/gdb/dwarf2/abbrev.c b/gdb/dwarf2/abbrev.c
index 59ff138b33d..b85018060fa 100644
--- a/gdb/dwarf2/abbrev.c
+++ b/gdb/dwarf2/abbrev.c
@@ -168,8 +168,9 @@ abbrev_table::read (struct objfile *objfile,
       cur_abbrev->attrs =
 	XOBNEWVEC (&abbrev_table->m_abbrev_obstack, struct attr_abbrev,
 		   cur_abbrev->num_attrs);
-      memcpy (cur_abbrev->attrs, cur_attrs.data (),
-	      cur_abbrev->num_attrs * sizeof (struct attr_abbrev));
+      if (!cur_attrs.empty ())
+	memcpy (cur_abbrev->attrs, cur_attrs.data (),
+		cur_abbrev->num_attrs * sizeof (struct attr_abbrev));
 
       abbrev_table->add_abbrev (abbrev_number, cur_abbrev);
 
diff --git a/gdbsupport/btrace-common.cc b/gdbsupport/btrace-common.cc
index 7d4f6424c82..e8b24db7d53 100644
--- a/gdbsupport/btrace-common.cc
+++ b/gdbsupport/btrace-common.cc
@@ -173,7 +173,8 @@ btrace_data_append (struct btrace_data *dst,
 	    size = src->variant.pt.size + dst->variant.pt.size;
 	    data = (gdb_byte *) xmalloc (size);
 
-	    memcpy (data, dst->variant.pt.data, dst->variant.pt.size);
+	    if (dst->variant.pt.size > 0)
+	      memcpy (data, dst->variant.pt.data, dst->variant.pt.size);
 	    memcpy (data + dst->variant.pt.size, src->variant.pt.data,
 		    src->variant.pt.size);
 
-- 
2.21.1

Re: [PATCH] Don't pass NULL to memcpy in gdb

[Simon Marchi]

On 2020-03-20 11:01 a.m., Tom Tromey wrote:
> I compiled gdb with -fsanitize=undefined and ran the test suite.
> 
> A couple of reports came from passing NULL to memcpy, e.g.:
> 
> [...]btrace-common.cc:176:13: runtime error: null pointer passed as argument 2, which is declared to never be null
> 
> While it would be better to fix this in the standard, in the meantime
> it seems easy to avoid this error.

I hope you're joking here :).  But otherwise, this LGTM.

Simon

Re: [PATCH] Don't pass NULL to memcpy in gdb

[Tom Tromey]

>>>>> "Simon" == Simon Marchi <simark@simark.ca> writes:

>> While it would be better to fix this in the standard, in the meantime
>> it seems easy to avoid this error.

Simon> I hope you're joking here :).  But otherwise, this LGTM.

I actually do think it would be better for the standard to change to say
that memcpy should accept NULL pointers provided that the length is 0.
Probably most implementations already work this way and it seems
harmless to allow.  It also seems relatively unlikely to change though.

Tom