Re: [PATCH 2/9] perf symbols: Add support for x86 .plt.sec

Namhyung Kim
2023-01-30T22:23:17Z

On Mon, Jan 30, 2023 at 10:35 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 30/01/23 19:34, Namhyung Kim wrote:
> > Hi Adrian,
> >
> > On Fri, Jan 27, 2023 at 9:02 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
> >>
> >> The section .plt.sec was originally added for MPX and was first called
> >> .plt.bnd. While MPX has been deprecated, .plt.sec is now also used for IBT.
> >> On x86_64, IBT seems to be enabled by default, but can be switched off
> >> using gcc option -fcf-protection=none. On 32-bit, option -z ibt will
> >> enable IBT.
> >>
> >> With .plt.sec, calls are made into .plt.sec instead of .plt, so it
> >> makes more sense to put the symbols there instead of .plt. A notable
> >> difference is that .plt.sec does not have a header entry.
> >>
> >> For x86, when synthesizing symbols for plt, use offset and entry size of
> >> .plt.sec instead of .plt when there is a .plt.sec section.
> >>
> >> Example on Ubuntu 22.04 gcc 11.3:
> >>
> >> Before:
> >>
> >> $ cat tstpltlib.c
> >> void fn1(void) {}
> >> void fn2(void) {}
> >> void fn3(void) {}
> >> void fn4(void) {}
> >> $ cat tstplt.c
> >> void fn1(void);
> >> void fn2(void);
> >> void fn3(void);
> >> void fn4(void);
> >>
> >> int main()
> >> {
> >> fn4();
> >> fn1();
> >> fn2();
> >> fn3();
> >> return 0;
> >> }
> >> $ gcc --version
> >> gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
> >> Copyright (C) 2021 Free Software Foundation, Inc.
> >> This is free software; see the source for copying conditions. There is NO
> >> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> >> $ gcc -Wall -Wextra -shared -o libtstpltlib.so tstpltlib.c
> >> $ gcc -Wall -Wextra -o tstplt tstplt.c -L . -ltstpltlib -Wl,-rpath=$(pwd)
> >> $ readelf -SW tstplt | grep 'plt\|Name'
> >> [Nr] Name Type Address Off Size ES Flg Lk Inf Al
> >> [11] .rela.plt RELA 0000000000000698 000698 000060 18 AI 6 24 8
> >> [13] .plt PROGBITS 0000000000001020 001020 000050 10 AX 0 0 16
> >> [14] .plt.got PROGBITS 0000000000001070 001070 000010 10 AX 0 0 16
> >> [15] .plt.sec PROGBITS 0000000000001080 001080 000040 10 AX 0 0 16
> >
> > On my machine, it's not enabled by default. And it doesn't create .plt.sec
> > even if I pass -fcf-protection=full option.
> >
> > $ gcc --version
> > gcc (Debian 12.2.0-10) 12.2.0
> > Copyright (C) 2022 Free Software Foundation, Inc.
> > This is free software; see the source for copying conditions. There is NO
> > warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> >
> > $ gcc -Wall -Wextra -shared -fcf-protection=full -o libtstplt.so tstpltlib.c
> > $ gcc -Wall -Wextra -fcf-protection=full -o tstplt tstplt.c -L.
> > -ltstpltlib -Wl,-rpath,$(pwd)
> > $ readelf -SW tstplt | grep 'plt\|Name'
> > [Nr] Name Type Address Off Size
> > ES Flg Lk Inf Al
> > [11] .rela.plt RELA 0000000000000688 000688
> > 000060 18 AI 6 24 8
> > [13] .plt PROGBITS 0000000000001020 001020
> > 000050 10 AX 0 0 16
> > [14] .plt.got PROGBITS 0000000000001070 001070
> > 000008 08 AX 0 0 8
> > [24] .got.plt PROGBITS 0000000000003fe8 002fe8
> > 000038 08 WA 0 0 8
>
> That is interesting. What does it say with -v i.e.
>
> gcc -v -Wall -Wextra -o tstplt tstplt.c -L . -ltstpltlib -Wl,-rpath=$(pwd)

Using built-in specs.
COLLECT_GCC=gcc
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GNU C17 (Debian 12.2.0-10) version 12.2.0 (x86_64-linux-gnu)
compiled by GNU C version 12.2.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.25-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 0bf64a455b69fb48d1b44a013a099136
COLLECT_GCC_OPTIONS='-Wall' '-Wextra' '-fcf-protection=full' '-v' '-o'
'tstplt' '-L.' '-mtune=generic' '-march=x86-64' '-dumpdir' 'tstplt-'
as -v --64 -o /tmp/cc0IMyNr.o /tmp/ccKPWeTD.s
GNU assembler version 2.39.50 (x86_64-linux-gnu) using BFD version
(GNU Binutils for Debian) 2.39.50.20221208
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'tstplt' '-L.' '-mtune=generic' '-march=x86-64' '-dumpdir' 'tstplt.'
/usr/lib/gcc/x86_64-linux-gnu/12/collect2 -plugin
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-L/usr/lib/gcc/x86_64-linux-gnu/12
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-L/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib
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>
> And what is the distribution?

It's a Debian (Testing) with some customization.

Thanks,
Namhyung