* -fpatchable-function-entry: leverage multi-byte NOP on x86
@ 2020-01-04 20:48 Fangrui Song
2020-01-06 8:15 ` Fangrui Song
2020-01-06 9:34 ` Martin Liška
0 siblings, 2 replies; 4+ messages in thread
From: Fangrui Song @ 2020-01-04 20:48 UTC (permalink / raw)
To: gcc; +Cc: Jan Hubicka
-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.)
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: -fpatchable-function-entry: leverage multi-byte NOP on x86
2020-01-04 20:48 -fpatchable-function-entry: leverage multi-byte NOP on x86 Fangrui Song
@ 2020-01-06 8:15 ` Fangrui Song
2020-01-06 9:34 ` Martin Liška
1 sibling, 0 replies; 4+ messages in thread
From: Fangrui Song @ 2020-01-06 8:15 UTC (permalink / raw)
To: gcc; +Cc: Jan Hubicka, Torsten Duwe
On 2020-01-04, 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>:
> 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.)
Archaeology is difficult with the mailing list archive...
CC Torsten as the patch author
https://gcc.gnu.org/ml/gcc-patches/2016-09/msg02257.html [PATCH v2] add -fprolog-pad=N option to c-family
https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00391.html [PATCH v12] add -fpatchable-function-entry=N,M option
Another finding is that
-fpatchable-function-entry=N[,M] is very similar to -mhotpatch=pre-halfwords,post-halfwords on S/390
https://gcc.gnu.org/onlinedocs/gcc/S_002f390-and-zSeries-Options.html
If the hotpatch option is enabled, a “hot-patching†function prologue
is generated for all functions in the compilation unit. The funtion
label is prepended with the given number of two-byte NOP instructions
(pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords
bytes are appended, using the largest NOP like instructions the
architecture allows (maximum 1000000).
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: -fpatchable-function-entry: leverage multi-byte NOP on x86
2020-01-04 20:48 -fpatchable-function-entry: leverage multi-byte NOP on x86 Fangrui Song
2020-01-06 8:15 ` Fangrui Song
@ 2020-01-06 9:34 ` Martin Liška
2020-01-06 9:40 ` Alexander Monakov
1 sibling, 1 reply; 4+ messages in thread
From: Martin Liška @ 2020-01-06 9:34 UTC (permalink / raw)
To: Fangrui Song, gcc; +Cc: Jan Hubicka
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
^ permalink raw reply [flat|nested] 4+ messages in thread
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2020-01-04 20:48 -fpatchable-function-entry: leverage multi-byte NOP on x86 Fangrui Song
2020-01-06 8:15 ` Fangrui Song
2020-01-06 9:34 ` Martin Liška
2020-01-06 9:40 ` Alexander Monakov
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