On 5 October 2012 17:01, Rob Herring robherring2@gmail.com wrote:
Here's a testcase. Compiled on ubuntu precise with "arm-linux-gnueabihf-gcc -O2 -marm -march=armv7-a test.c".
typedef unsigned short u16; typedef unsigned short __sum16; typedef unsigned int __u32; typedef unsigned char __u8; typedef __u32 __be32; typedef u16 __be16;
struct iphdr { __u8 ihl:4, version:4; __u8 tos; __be16 tot_len; __be16 id; __be16 frag_off; __u8 ttl; __u8 protocol; __sum16 check; __be32 saddr; __be32 daddr; /*The options start here. */ };
#define ntohl(x) __swab32((__u32)(__be32)(x)) #define IP_DF 0x4000 /* Flag: "Don't Fragment" */
static inline __attribute__((const)) __u32 __swab32(__u32 x) { __asm__ ("rev %0, %1" : "=r" (x) : "r" (x)); return x; }
int main(void * buffer, unsigned int *p_id) { unsigned int id; int flush = 1; const struct iphdr *iph = buffer; __u32 len = *p_id;
id = ntohl(*(__be32 *)&iph->id); flush = (u16)((ntohl(*(__be32 *)iph) ^ len) | (id ^ IP_DF)); id >>= 16; *p_id = id; return flush;}
The problem is the (__be32 *) casts. This is a normal pointer to a 32-bit, which is assumed to be aligned, and the cast overrides the packed attribute from the struct. Dereferencing these cast expressions must be done with the macros from asm/unaligned.h