On Wed, 25 Mar 2009 01:13:03 +0100
Ralf Baechle <ralf@xxxxxxxxxxxxxx> wrote:
> In fs/read_write.c:
>
> SYSCALL_DEFINE5(llseek, unsigned int, fd, unsigned long, offset_high,
> unsigned long, offset_low, loff_t __user *, result,
> unsigned int, origin)
> ...
> offset = vfs_llseek(file, ((loff_t) offset_high << 32) | offset_low,
> origin);
>
> On a 64-bit system that define CONFIG_HAVE_SYSCALL_WRAPPERS SYSCALL_DEFINEx
> will truncate long arguments to 32-bit and on some architectures such as
> MIPS sign-extended to 64-bit again. On such architectures passing a
> value with bit 31 in offset_low set will result in a huge 64-bit offset
> being passed to vfs_llseek() and it failiing with EINVAL.
How is that possible? If you have CONFIG_HAVE_SYSCALL_WRAPPERS defined
then the wrapper will (in this case) cast offset_low from long to
unsigned long. It won't truncate or sign extend anything here.
The whole operation should be a NOP.
This is what you get after macro expansion (BUILD_BUG_ON removed):
long sys_llseek(unsigned int fd, unsigned long offset_high, unsigned long offset_low, loff_t * result, unsigned int origin);
static inline __attribute__((always_inline))
long SYSC_llseek(unsigned int fd, unsigned long offset_high, unsigned long offset_low, loff_t * result, unsigned int origin);
long SyS_llseek(long fd, long offset_high, long offset_low, long result, long origin)
{
return (long) SYSC_llseek((unsigned int) fd, (unsigned long) offset_high, (unsigned long) offset_low, (loff_t *) result, (unsigned int) origin);
}
asm ("\t.globl " "sys_llseek" "\n\t.set " "sys_llseek" ", " "SyS_llseek");
static inline __attribute__((always_inline))
long SYSC_llseek(unsigned int fd, unsigned long offset_high, unsigned long offset_low, loff_t * result, unsigned int origin)
{
[...]
}
