On 06/26/2016, 07:17 PM, Linus Torvalds wrote:
> On Sun, Jun 26, 2016 at 2:24 AM, Vegard Nossum <vegard.nos...@gmail.com> 
> wrote:
>>
>> This is the best I could come up with: assuming gcc is not allowed to
>> reason about what's inside the asm(), this is the only way I could
>> think of to lose the array information without incurring unnecessary
>> overheads. It should also be relatively safe as there is no way to
>> accidentally use the underlying arrays without explicitly declaring
>> them.
> 
> Ugh. I worry about the other places where we do things like this,
> depending on the linker just assigning the addresses and us being able
> to compare them.
> 
> If there is a compiler option to disable this optimization, I would
> almost prefer that.. Because we really do have a whole slew of these
> things.

Any update on this? Couple months later and I still hit this.

Quick checking shows, that a lot code depends on comparing two arrays
(undefined behaviour):
ftrace_init
  count = __stop_mcount_loc - __start_mcount_loc;
tracer_alloc_buffers
  if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)


FWIW this indeed fixes the get_builtin_firmware case for me:
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -97,9 +97,11 @@ extern struct builtin_fw __end_builtin_fw[];
 bool get_builtin_firmware(struct cpio_data *cd, const char *name)
 {
 #ifdef CONFIG_FW_LOADER
-       struct builtin_fw *b_fw;
+       struct builtin_fw *b_fw = __start_builtin_fw;

-       for (b_fw = __start_builtin_fw; b_fw != __end_builtin_fw; b_fw++) {
+       OPTIMIZER_HIDE_VAR(b_fw);
+
+       for (; b_fw != __end_builtin_fw; b_fw++) {
                if (!strcmp(name, b_fw->name)) {
                        cd->size = b_fw->size;
                        cd->data = b_fw->data;



What about adding:
#define for_each_vmlinux_symbol(sym, start, stop) \
  for (sym = start, OPTIMIZER_HIDE_VAR(sym); sym != stop; sym++)

and converting at least the iterators?

What to do with the array subtractions and comparisons (like tracing), I
don't know (yet).

thanks,
-- 
js
suse labs

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