On Tue, Dec 22, 2015 at 11:38:07AM -0800, Tony Luck wrote:
> I interpreted that comment as "stop playing with %rax in the fault
> handler ... just change the IP to point the the .fixup location" ...
> the target of the fixup being the "landing pad".
>
> Right now this function has only one set of fault fixups (for machine
> checks). When I tackle copy_from_user() it will sprout a second
> set for page faults, and then will look a bit more like Andy's dual
> landing pad example.
>
> I still need an indicator to the caller which type of fault happened
> since their actions will be different. So BIT(63) lives on ... but is
> now set in the .fixup section rather than in the machine check
> code.
You mean this previous example of yours:
int copy_from_user(void *to, void *from, unsigned long n)
{
u64 ret = mcsafe_memcpy(to, from, n);
if (COPY_HAD_MCHECK(r)) {
if (memory_failure(COPY_MCHECK_PADDR(ret) >> PAGE_SIZE, ...))
force_sig(SIGBUS, current);
return something;
} else
return ret;
}
?
So what's wrong with mcsafe_memcpy() returning a proper retval which
says what type of fault happened?
I know, memcpy returns the ptr to @dest like a parrot but your version
mcsafe_memcpy() will be different. It can even be called __mcsafe_memcpy
and have a wrapper around it which fiddles out the proper retvals and
returns @dest after all. It would still be cleaner this way IMHO.
> I'll move the function and #defines as you suggest - we don't need
> new files for these. Also will fix the assembly code.
> [In my defense that load immediate 0x8000000000000000 and 'or'
> was what gcc -O2 generates from a simple bit of C code to set
> bit 63 ... perhaps it is faster, or perhaps gcc is on drugs. In this
> case code compactness wins over possible speed difference].
Well, upon a second thought, the reason why gcc would use that huge
immediate could be because by using BTS, it clobbers the carry flag
in rFLAGS. And I guess we don't want that. Although any Jcc or other
conditional instructions touching rFLAGS following will overwrite that
bit so it won't really matter.
I've asked a gcc person, we'll see what interesting explanation comes
back.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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