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

Fangrui Song i@maskray.me
Sat Jan 4 20:48:00 GMT 2020


-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>:
    0:   90                      nop
    1:   90                      nop
    2:   90                      nop
    3:   90                      nop
    4:   90                      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
    f:   c3                      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.)



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