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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 may be enabled by default, but can be switched off
using gcc option -fcf-protection=none, or switched on by -z ibt or -z
ibtplt. On 32-bit, option -z ibt or -z ibtplt 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 -z ibt -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
$ perf record -e intel_pt//u --filter 'filter main @ ./tstplt' ./tstplt
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.015 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
38970.522546686: tr strt 0 [unknown] => 55fc222a81a9 main+0x0
38970.522546686: tr end call 55fc222a81b1 main+0x8 => 55fc222a80a0 [unknown]
38970.522546687: tr strt 0 [unknown] => 55fc222a81b6 main+0xd
38970.522546687: tr end call 55fc222a81b6 main+0xd => 55fc222a8080 [unknown]
38970.522546688: tr strt 0 [unknown] => 55fc222a81bb main+0x12
38970.522546688: tr end call 55fc222a81bb main+0x12 => 55fc222a80b0 [unknown]
38970.522546688: tr strt 0 [unknown] => 55fc222a81c0 main+0x17
38970.522546688: tr end call 55fc222a81c0 main+0x17 => 55fc222a8090 [unknown]
38970.522546689: tr strt 0 [unknown] => 55fc222a81c5 main+0x1c
38970.522546894: tr end return 55fc222a81cb main+0x22 => 7f3a4dc29d90 __libc_start_call_main+0x80
After:
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
38970.522546686: tr strt 0 [unknown] => 55fc222a81a9 main+0x0
38970.522546686: tr end call 55fc222a81b1 main+0x8 => 55fc222a80a0 fn4@plt+0x0
38970.522546687: tr strt 0 [unknown] => 55fc222a81b6 main+0xd
38970.522546687: tr end call 55fc222a81b6 main+0xd => 55fc222a8080 fn1@plt+0x0
38970.522546688: tr strt 0 [unknown] => 55fc222a81bb main+0x12
38970.522546688: tr end call 55fc222a81bb main+0x12 => 55fc222a80b0 fn2@plt+0x0
38970.522546688: tr strt 0 [unknown] => 55fc222a81c0 main+0x17
38970.522546688: tr end call 55fc222a81c0 main+0x17 => 55fc222a8090 fn3@plt+0x0
38970.522546689: tr strt 0 [unknown] => 55fc222a81c5 main+0x1c
38970.522546894: tr end return 55fc222a81cb main+0x22 => 7f3a4dc29d90 __libc_start_call_main+0x80
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230131131625.6964-3-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.