On Fri, Oct 03, 2014 at 07:34:34AM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2014-10-02 at 15:41 +0100, Romeo Cane wrote:
> > Declaring sys_call_table as a pointer causes the compiler to generate the
> > wrong lookup code in arch_syscall_addr
>
> Care to elaborate ?
>
> Ben.
>
> > Signed-off-by: Romeo Cane <romeo.cane....@coriant.com>
> > ---
> > arch/powerpc/include/asm/syscall.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/include/asm/syscall.h
> > b/arch/powerpc/include/asm/syscall.h
> > index b54b2ad..528ba9d 100644
> > --- a/arch/powerpc/include/asm/syscall.h
> > +++ b/arch/powerpc/include/asm/syscall.h
> > @@ -17,7 +17,7 @@
> >
> > /* ftrace syscalls requires exporting the sys_call_table */
> > #ifdef CONFIG_FTRACE_SYSCALLS
> > -extern const unsigned long *sys_call_table;
> > +extern const unsigned long sys_call_table[];
> > #endif /* CONFIG_FTRACE_SYSCALLS */
> >
> > static inline long syscall_get_nr(struct task_struct *task,
>
>
Hi Ben,
this is the arch_syscall_addr function from kernel/trace/trace_syscalls.c:
unsigned long __init __weak arch_syscall_addr(int nr)
{
return (unsigned long)sys_call_table[nr];
}
on my platform (E500MC) the generated assembly code is as follows:
without the patch:
<arch_syscall_addr>:
lis r9,-16384
rlwinm r3,r3,2,0,29
lwz r11,30640(r9)
lwzx r3,r11,r3
blr
with the patch:
<arch_syscall_addr>:
lis r9,-16384
rlwinm r3,r3,2,0,29
addi r9,r9,30640
lwzx r3,r9,r3
blr
the goal of the function is to retrieve the n-th element of the table (i.e. the
address of a syscall)
Without the patch, the returned value is in fact the memory content pointed by
the address of the first syscall plus an offset, that is not what we want.
The consequence is that ftrace of syscalls doesn't work.
That table has always been declared as a pointer since the support for syscalls
tracing has been introduced for powerpc years ago, so I'm wondering why nobody
else had this problem before.
Other architectures are not affected since in their includes the table is
already declared as an array.
Romeo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
