[PATCH] Avoid buffer overflow in value_x_unop

[Tom Tromey]

Commit 6b1747cd1 ("invoke_xmethod & array_view") contains this change:

- argvec = (struct value **) alloca (sizeof (struct value *) * 4);
+ value *argvec_storage[3];
+ gdb::array_view<value *> argvec = argvec_storage;

However, value_x_unop still does:

argvec[2] = value_from_longest (builtin_type (gdbarch)->builtin_int, 0);
argvec[3] = 0;

This triggers an error with -fsanitize=address from userdef.exp:

ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffdcf185068 at pc 0x000000e4f912 bp 0x7ffdcf184d80 sp 0x7ffdcf184d70
WRITE of size 8 at 0x7ffdcf185068 thread T0
#0 0xe4f911 in value_x_unop(value*, exp_opcode, noside) ../../binutils-gdb/gdb/valarith.c:557
[...]

I think the two assignments to argvec[3] should just be removed, and
that this was intended in the earlier patch but just missed.

This passes userdef.exp with -fsanitize=address.

gdb/ChangeLog
2018-11-28 Tom Tromey <***@tromey.com>

* valarith.c (value_x_unop): Don't set argvec[3].
---
gdb/ChangeLog | 4 ++++
gdb/valarith.c | 2 --
2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/gdb/valarith.c b/gdb/valarith.c
index 3a59ada2d5..66cd9042d4 100644
--- a/gdb/valarith.c
+++ b/gdb/valarith.c
@@ -554,13 +554,11 @@ value_x_unop (struct value *arg1, enum exp_opcode op, enum noside noside)
case UNOP_POSTINCREMENT:
strcpy (ptr, "++");
argvec[2] = value_from_longest (builtin_type (gdbarch)->builtin_int, 0);
- argvec[3] = 0;
nargs ++;
break;
case UNOP_POSTDECREMENT:
strcpy (ptr, "--");
argvec[2] = value_from_longest (builtin_type (gdbarch)->builtin_int, 0);
- argvec[3] = 0;
nargs ++;
break;
case UNOP_LOGICAL_NOT:

[Pedro Alves]

Post by Tom Tromey
- argvec = (struct value **) alloca (sizeof (struct value *) * 4);
+ value *argvec_storage[3];
+ gdb::array_view<value *> argvec = argvec_storage;
argvec[2] = value_from_longest (builtin_type (gdbarch)->builtin_int, 0);
argvec[3] = 0;
ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffdcf185068 at pc 0x000000e4f912 bp 0x7ffdcf184d80 sp 0x7ffdcf184d70
WRITE of size 8 at 0x7ffdcf185068 thread T0
#0 0xe4f911 in value_x_unop(value*, exp_opcode, noside) ../../binutils-gdb/gdb/valarith.c:557
[...]
I think the two assignments to argvec[3] should just be removed, and
that this was intended in the earlier patch but just missed.

Indeed it was. Sorry about that.

Post by Tom Tromey
This passes userdef.exp with -fsanitize=address.
gdb/ChangeLog
* valarith.c (value_x_unop): Don't set argvec[3].

OK.

Thanks,
Pedro Alves