-fpatchable-function-entry: leverage multi-byte NOP on x86

[Martin Liška]

On 1/4/20 9:48 PM, Fangrui Song wrote:
> -fpatchable-function-entry is used by Linux kernel arm64/parisc. The
> feature works for x86, but does not leverage multi-byte NOP.
> 
> % cat a.c
> int foo() { return 0; }
> % gcc -fpatchable-function-entry=5,0 -c a.c
> % objdump -d a.o
> ...
> 0000000000000000 <foo>:
>  Â Â  0:Â Â  90Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  nop
>  Â Â  1:Â Â  90Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  nop
>  Â Â  2:Â Â  90Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  nop
>  Â Â  3:Â Â  90Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  nop
>  Â Â  4:Â Â  90Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  nop
>     5:   55                      push   %rbp
>     6:   48 89 e5                mov    %rsp,%rbp
>     9:   b8 00 00 00 00          mov    $0x0,%eax
>     e:   5d                      pop    %rbp
>  Â Â  f:Â Â  c3Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  retq
> 
> 
> (I am not sure who should be notified. I CCed x86-64-port maintainer
> (Jan Hubicka <hubicka@ucw.cz>), as listed on gcc/MAINTAINERS.)

Hello.

You are right, we do not leverage multi-byte NOPs. Note that the support depends
on a CPU model (-march) and the similar code is quite complex in binutils:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gas/config/tc-i386.c;h=d0b8f2624a1885d83d2595474bfd78ae844f48f2;hb=HEAD#l1441

I'm not sure how worthy would it be to implement that?
Thanks,
Martin