diff mbox

Avoid invoking undefined behavior when initializing CRC table

Message ID alpine.DEB.2.20.9.1512272315120.19987@idea
State New, archived
Headers

Commit Message

Patrick Palka Dec. 28, 2015, 4:15 a.m. UTC 
  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

Pedro Alves Dec. 28, 2015, 11:53 a.m. UTC | #1 
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
diff mbox

Patch

diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 70acafc..0e3ac4e 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -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;