On Wed, Mar 09, 2016 at 12:52:03AM +1100, Balbir Singh wrote:
>
> On 08/03/16 21:45, Torsten Duwe wrote:
> > To be fair, my last mail still was not 100% correct, but the conclusion
Wrote a correction to the correction. It should be clear now. Please nag me
if it isn't clear why klp_return_helper and its stack frame is needed.
> > that the mini frame is not needed at all is invalid. Please leave it as it
> > was, I'm working on a test / demonstrator for how to handle these.
> Why, the magic will be in the patched function? Please share the test/demonstrator
Here it comes...
> > NAKed-by: Torsten Duwe <duwe@xxxxxxx>
> Why? For using CR+4 or removing the frame? Or you believe there is a better way to
> handle this that work, IOW what is broken?
The stack frame removal. You're risking a memory access or jump into nirvana
or and endless loop.
klp_return_helper will do the right thing, and functions like e.g. printk
I would live patch like this (untested :-) :
------------------------8<----------------------
#include <stdarg.h>
/* compile using "-ffixed-r14"! */
register unsigned long pass_TOC asm("r14");
/*
* Function pre-prologue to pop the klp_return_helper
* mini stack frame. The saved r2 TOC value is read and
* passed in pass_TOC (r14), the original LR is passed
* in r0 and the LR itself. R12 is updated appropriately
* for local TOC recalculation.
*/
extern void caller(void) asm("printk");
void caller(void)
{
asm("ld %0,24(1)" : "=r" (pass_TOC));
asm("addi 1,1,32");
asm("addi 12,12,(real_printk-printk)@l");
asm("ld 0,16(1)\n\tmtlr 0");
asm("b real_printk");
}
extern int vprintk_default(const char *fmt, va_list args);
extern int printk(const char *fmt, ...) asm("real_printk");
int printk(const char *fmt, ...)
{
va_list args;
int r;
va_start(args, fmt);
r = vprintk_default(fmt, args);
va_end(args);
asm("mr 2,%0" : : "r" (pass_TOC));
return r;
}
------------------------8<----------------------
Signed-off-by: Torsten Duwe <duwe@xxxxxxx>
As you can see, the extra effort for args on the stack is limited.
Torsten
--
To unsubscribe from this list: send the line "unsubscribe live-patching" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
