[PATCH] Allocate buffer with proper size in amd64_pseudo_register_{read_value,write}

Simon Marchi
2018-10-21T02:59:55Z

Running "maintenance selftest" on an amd64 build with AddressSanitizer
enabled, I get this:

==18126==ERROR: AddressSanitizer: dynamic-stack-buffer-overflow on address 0x7ffdf72397c1 at pc 0x7fb5f437b011 bp 0x7ffdf7239740 sp 0x7ffdf7238ee8
WRITE of size 8 at 0x7ffdf72397c1 thread T0
#0 0x7fb5f437b010 in __interceptor_memcpy /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:737
#1 0x55a1f899c1b3 in readable_regcache::raw_read(int, unsigned char*) /home/simark/src/binutils-gdb/gdb/regcache.c:530
#2 0x55a1f7db241b in amd64_pseudo_register_read_value /home/simark/src/binutils-gdb/gdb/amd64-tdep.c:384
#3 0x55a1f8413a2e in gdbarch_pseudo_register_read_value(gdbarch*, readable_regcache*, int) /home/simark/src/binutils-gdb/gdb/gdbarch.c:1992
#4 0x55a1f899c9d1 in readable_regcache::cooked_read(int, unsigned char*) /home/simark/src/binutils-gdb/gdb/regcache.c:636
#5 0x55a1f89a2251 in cooked_read_test /home/simark/src/binutils-gdb/gdb/regcache.c:1649

In amd64_pseudo_register_read_value, when we try to read the al
register, for example, we need to read rax and extract al from it. We
allocate a buffer of the size of al (1 byte):

gdb_byte *raw_buf = (gdb_byte *) alloca (register_size (gdbarch, regnum));

but read in it the whole rax value (8 bytes):

status = regcache->raw_read (gpnum, raw_buf);

Fix it by allocating a buffer correctly sized for the full register from
which the smaller register is extracted. The
amd64_pseudo_register_write function had the same problem.

gdb/ChangeLog:

* amd64-tdep.c (amd64_pseudo_register_read_value): Use
correctly-sized buffer with raw_read.
(amd64_pseudo_register_write): Use correctly-sized buffer for
raw_read/raw_write.
---
gdb/amd64-tdep.c | 31 ++++++++++++++++++-------------
1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 088542d72b49..abf3e4d91904 100644
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -352,16 +352,12 @@ amd64_pseudo_register_read_value (struct gdbarch *gdbarch,
readable_regcache *regcache,
int regnum)
{
- gdb_byte *raw_buf = (gdb_byte *) alloca (register_size (gdbarch, regnum));
struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
- enum register_status status;
- struct value *result_value;
- gdb_byte *buf;

- result_value = allocate_value (register_type (gdbarch, regnum));
+ value *result_value = allocate_value (register_type (gdbarch, regnum));
VALUE_LVAL (result_value) = lval_register;
VALUE_REGNUM (result_value) = regnum;
- buf = value_contents_raw (result_value);
+ gdb_byte *buf = value_contents_raw (result_value);

if (i386_byte_regnum_p (gdbarch, regnum))
{
@@ -370,9 +366,11 @@ amd64_pseudo_register_read_value (struct gdbarch *gdbarch,
/* Extract (always little endian). */
if (gpnum >= AMD64_NUM_LOWER_BYTE_REGS)
{
+ gpnum -= AMD64_NUM_LOWER_BYTE_REGS;
+ gdb_byte raw_buf[register_size (gdbarch, gpnum)];
+
/* Special handling for AH, BH, CH, DH. */
- status = regcache->raw_read (gpnum - AMD64_NUM_LOWER_BYTE_REGS,
- raw_buf);
+ register_status status = regcache->raw_read (gpnum, raw_buf);
if (status == REG_VALID)
memcpy (buf, raw_buf + 1, 1);
else
@@ -381,7 +379,8 @@ amd64_pseudo_register_read_value (struct gdbarch *gdbarch,
}
else
{
- status = regcache->raw_read (gpnum, raw_buf);
+ gdb_byte raw_buf[register_size (gdbarch, gpnum)];
+ register_status status = regcache->raw_read (gpnum, raw_buf);
if (status == REG_VALID)
memcpy (buf, raw_buf, 1);
else
@@ -392,8 +391,9 @@ amd64_pseudo_register_read_value (struct gdbarch *gdbarch,
else if (i386_dword_regnum_p (gdbarch, regnum))
{
int gpnum = regnum - tdep->eax_regnum;
+ gdb_byte raw_buf[register_size (gdbarch, gpnum)];
/* Extract (always little endian). */
- status = regcache->raw_read (gpnum, raw_buf);
+ register_status status = regcache->raw_read (gpnum, raw_buf);
if (status == REG_VALID)
memcpy (buf, raw_buf, 4);
else
@@ -412,7 +412,6 @@ amd64_pseudo_register_write (struct gdbarch *gdbarch,
struct regcache *regcache,
int regnum, const gdb_byte *buf)
{
- gdb_byte *raw_buf = (gdb_byte *) alloca (register_size (gdbarch, regnum));
struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);

if (i386_byte_regnum_p (gdbarch, regnum))
@@ -421,15 +420,20 @@ amd64_pseudo_register_write (struct gdbarch *gdbarch,

if (gpnum >= AMD64_NUM_LOWER_BYTE_REGS)
{
+ gpnum -= AMD64_NUM_LOWER_BYTE_REGS;
+ gdb_byte raw_buf[register_size (gdbarch, gpnum)];
+
/* Read ... AH, BH, CH, DH. */
- regcache->raw_read (gpnum - AMD64_NUM_LOWER_BYTE_REGS, raw_buf);
+ regcache->raw_read (gpnum, raw_buf);
/* ... Modify ... (always little endian). */
memcpy (raw_buf + 1, buf, 1);
/* ... Write. */
- regcache->raw_write (gpnum - AMD64_NUM_LOWER_BYTE_REGS, raw_buf);
+ regcache->raw_write (gpnum, raw_buf);
}
else
{
+ gdb_byte raw_buf[register_size (gdbarch, gpnum)];
+
/* Read ... */
regcache->raw_read (gpnum, raw_buf);
/* ... Modify ... (always little endian). */
@@ -441,6 +445,7 @@ amd64_pseudo_register_write (struct gdbarch *gdbarch,
else if (i386_dword_regnum_p (gdbarch, regnum))
{
int gpnum = regnum - tdep->eax_regnum;
+ gdb_byte raw_buf[register_size (gdbarch, gpnum)];

/* Read ... */
regcache->raw_read (gpnum, raw_buf);
--
2.19.1