From: Patrick Palka <patrick@parcs.ath.cx>
To: Patrick Palka <patrick@parcs.ath.cx>
Cc: gdb-patches@sourceware.org
Subject: Re: [PATCH] Avoid invoking undefined behavior when initializing CRC table
Date: Mon, 28 Dec 2015 04:15:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.20.9.1512272315120.19987@idea> (raw)
In-Reply-To: <1451275797-648-1-git-send-email-patrick@parcs.ath.cx>
[-- Attachment #1: Type: text/plain, Size: 2423 bytes --]
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 âcrc32â:
> .../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:
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;
next prev parent reply other threads:[~2015-12-28 4:15 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-28 4:10 Patrick Palka
2015-12-28 4:15 ` Patrick Palka [this message]
2015-12-28 11:53 ` Pedro Alves
2015-12-28 16:22 ` [PATCH] [COMMITTED] Use libiberty's crc32 implementation in gdbserver Patrick Palka
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