Avoid invoking undefined behavior when initializing CRC table
Commit Message
On Sun, 27 Dec 2015, Patrick Palka wrote:
> When I built GDB with (an older snapshot of) GCC 6 I get the following
> error:
>
> .../binutils-gdb/gdb/gdbserver/server.c: In function ‘crc32’:
> .../binutils-gdb/gdb/gdbserver/server.c:1895:15: error: iteration 128 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
> for (c = i << 24, j = 8; j > 0; --j)
> ^
> .../binutils-gdb/gdb/gdbserver/server.c:1893:7: note: within this loop
> for (i = 0; i < 256; i++)
> ^
> This error seems to be correct. When the variable "int i" is >= 128,
> the computation "i << 24" overflows for 32-bit signed int.
>
> To avoid shifting into the sign bit, this patch makes the variables i
> (and j, because why not) have type unsigned int instead.
>
> (Alternatively, I can just define this local crc32 function in terms of
> libiberty's xcrc32. Any reason not to? xcrc32 seems to be
> based off of GDB's crc32 implementation. Its documentation even
> refers to it!)
And here's a rough diff that defines crc32 in terms of xcrc32:
Comments
On 12/28/2015 04:15 AM, Patrick Palka wrote:
> On Sun, 27 Dec 2015, Patrick Palka wrote:
>
>> When I built GDB with (an older snapshot of) GCC 6 I get the following
>> error:
>>
>> .../binutils-gdb/gdb/gdbserver/server.c: In function ‘crc32’:
>> .../binutils-gdb/gdb/gdbserver/server.c:1895:15: error: iteration 128 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
>> for (c = i << 24, j = 8; j > 0; --j)
>> ^
>> .../binutils-gdb/gdb/gdbserver/server.c:1893:7: note: within this loop
>> for (i = 0; i < 256; i++)
>> ^
>> This error seems to be correct. When the variable "int i" is >= 128,
>> the computation "i << 24" overflows for 32-bit signed int.
>>
>> To avoid shifting into the sign bit, this patch makes the variables i
>> (and j, because why not) have type unsigned int instead.
>>
>> (Alternatively, I can just define this local crc32 function in terms of
>> libiberty's xcrc32. Any reason not to?
Just history. gdbserver only started linking with libiberty in
2014 (0b04e52316) and the gdbserver crc32 code predates that.
> xcrc32 seems to be
>> based off of GDB's crc32 implementation. Its documentation even
>> refers to it!)
>
> And here's a rough diff that defines crc32 in terms of xcrc32:
>
That looks good. Tom did the same on the GDB side in 85ec6ce7d5 (2013).
Just in case, please make sure testing against gdbserver doesn't regress:
$ make check -j8 RUNTESTFLAGS="--target_board=native-gdbserver" FORCE_PARALLEL=1
particularly, the compare-sections command. We have a test which covers it:
$ make check RUNTESTFLAGS="--target_board=native-gdbserver compare-sections.exp"
Thanks,
Pedro Alves
@@ -1911,11 +1911,6 @@ handle_qxfer (char *own_buf, int packet_len, int *new_packet_len_p)
return 0;
}
-/* Table used by the crc32 function to calcuate the checksum. */
-
-static unsigned int crc32_table[256] =
-{0, 0};
-
/* Compute 32 bit CRC from inferior memory.
On success, return 32 bit CRC.
@@ -1924,20 +1919,6 @@ static unsigned int crc32_table[256] =
static unsigned long long
crc32 (CORE_ADDR base, int len, unsigned int crc)
{
- if (!crc32_table[1])
- {
- /* Initialize the CRC table and the decoding table. */
- unsigned int i, j;
- unsigned int c;
-
- for (i = 0; i < 256; i++)
- {
- for (c = i << 24, j = 8; j > 0; --j)
- c = c & 0x80000000 ? (c << 1) ^ 0x04c11db7 : (c << 1);
- crc32_table[i] = c;
- }
- }
-
while (len--)
{
unsigned char byte = 0;
@@ -1946,7 +1927,7 @@ crc32 (CORE_ADDR base, int len, unsigned int crc)
if (read_inferior_memory (base, &byte, 1) != 0)
return (unsigned long long) -1;
- crc = (crc << 8) ^ crc32_table[((crc >> 24) ^ byte) & 255];
+ crc = xcrc32 (&byte, 1, crc);
base++;
}
return (unsigned long long) crc;