On Thu, Jun 9, 2022 at 4:27 PM James Hilliard [off-list ref] wrote:

On Thu, Jun 9, 2022 at 12:13 PM Andrii Nakryiko
[off-list ref] wrote:

quoted

On Wed, Jun 8, 2022 at 11:24 PM James Hilliard
[off-list ref] wrote:

quoted

It seems the gcc preprocessor breaks unless pragmas are wrapped
individually inside macros when surrounding __attribute__.

Fixes errors like:
error: expected identifier or '(' before '#pragma'
  106 | SEC("cgroup/bind6")
      | ^~~

error: expected '=', ',', ';', 'asm' or '__attribute__' before '#pragma'
  114 | char _license[] SEC("license") = "GPL";
      | ^~~

Signed-off-by: James Hilliard <redacted>
---
Changes v2 -> v3:
  - just fix SEC pragma
Changes v1 -> v2:
  - replace typeof with __typeof__ instead of changing pragma macros
---
 tools/lib/bpf/bpf_helpers.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index fb04eaf367f1..66d23c47c206 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h

@@ -22,11 +22,12 @@
  * To allow use of SEC() with externs (e.g., for extern .maps declarations),
  * make sure __attribute__((unused)) doesn't trigger compilation warning.
  */
+#define DO_PRAGMA(x) _Pragma(#x)
 #define SEC(name) \
-       _Pragma("GCC diagnostic push")                                      \
-       _Pragma("GCC diagnostic ignored \"-Wignored-attributes\"")          \
+       DO_PRAGMA("GCC diagnostic push")                                    \
+       DO_PRAGMA("GCC diagnostic ignored \"-Wignored-attributes\"")        \
        __attribute__((section(name), used))                                \
-       _Pragma("GCC diagnostic pop")                                       \
+       DO_PRAGMA("GCC diagnostic pop")                                     \

I'm not going to accept this unless I can repro it in the first place.
Using -std=c17 doesn't trigger such issue. Please provide the repro
first. Building systemd is not a repro, unfortunately. Please try to
do it based on libbpf-bootstrap ([0])

  [0] https://github.com/libbpf/libbpf-bootstrap

Seems to reproduce just fine already there with:
https://github.com/libbpf/libbpf-bootstrap/blob/31face36d469a0e3e4c4ac1cafc66747d3150930/examples/c/minimal.bpf.c

See here:
$ /home/buildroot/buildroot/output/per-package/libbpf/host/bin/bpf-gcc
-Winline -O2 -mframe-limit=32767 -mco-re -gbtf -std=gnu17 -v
-D__x86_64__ -mlittle-endian -I
/home/buildroot/buildroot/output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include
minimal.bpf.c -o minimal.bpf.o
Using built-in specs.
COLLECT_GCC=/home/buildroot/buildroot/output/per-package/libbpf/host/bin/bpf-gcc.br_real
COLLECT_LTO_WRAPPER=/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../libexec/gcc/bpf-buildroot-none/12.1.0/lto-wrapper
Target: bpf-buildroot-none
Configured with: ./configure
--prefix=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host
--sysconfdir=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host/etc
--localstatedir=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host/var
--enable-shared --disable-static --disable-gtk-doc
--disable-gtk-doc-html --disable-doc --disable-docs
--disable-documentation --disable-debug --with-xmlto=no --with-fop=no
--disable-nls --disable-dependency-tracking
--target=bpf-buildroot-none
--prefix=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host
--sysconfdir=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host/etc
--enable-languages=c --with-gnu-ld --enable-static
--disable-decimal-float --disable-gcov --disable-libssp
--disable-multilib --disable-shared
--with-gmp=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host
--with-mpc=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host
--with-mpfr=/home/buildroot/buildroot/output/per-package/host-gcc-bpf/host
--with-pkgversion='Buildroot 2022.05-118-ge052166011-dirty'
--with-bugurl=http://bugs.buildroot.net/ --without-zstd --without-isl
--without-cloog
Thread model: single
Supported LTO compression algorithms: zlib
gcc version 12.1.0 (Buildroot 2022.05-118-ge052166011-dirty)
COLLECT_GCC_OPTIONS='--sysroot=/home/buildroot/buildroot/output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot'
'-Winline' '-O2' '-mframe-limit=32767' '-mco-re' '-gbtf' '-std=gnu17'
'-v' '-D' '__x86_64__' '-mlittle-endian' '-I'
'/home/buildroot/buildroot/output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include'
'-o' 'minimal.bpf.o' '-dumpdir' 'minimal.bpf.o-'
 /home/buildroot/buildroot/output/per-package/libbpf/host/bin/../libexec/gcc/bpf-buildroot-none/12.1.0/cc1
-quiet -v -I /home/buildroot/buildroot/output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include
-iprefix /home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/bpf-buildroot-none/12.1.0/
-isysroot /home/buildroot/buildroot/output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot
-D __x86_64__ minimal.bpf.c -quiet -dumpdir minimal.bpf.o- -dumpbase
minimal.bpf.c -dumpbase-ext .c -mframe-limit=32767 -mco-re
-mlittle-endian -gbtf -O2 -Winline -std=gnu17 -version -o
/tmp/cct4AXvg.s
GNU C17 (Buildroot 2022.05-118-ge052166011-dirty) version 12.1.0
(bpf-buildroot-none)
    compiled by GNU C version 12.1.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/bpf-buildroot-none/12.1.0/../../../../bpf-buildroot-none/sys-include"
ignoring nonexistent directory
"/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/bpf-buildroot-none/12.1.0/../../../../bpf-buildroot-none/include"
ignoring duplicate directory
"/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/../../lib/gcc/bpf-buildroot-none/12.1.0/include"
ignoring duplicate directory
"/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/../../lib/gcc/bpf-buildroot-none/12.1.0/include-fixed"
ignoring nonexistent directory
"/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/../../lib/gcc/bpf-buildroot-none/12.1.0/../../../../bpf-buildroot-none/sys-include"
ignoring nonexistent directory
"/home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/../../lib/gcc/bpf-buildroot-none/12.1.0/../../../../bpf-buildroot-none/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/buildroot/buildroot/output/per-package/libbpf/host/x86_64-buildroot-linux-gnu/sysroot/usr/include
 /home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/bpf-buildroot-none/12.1.0/include
 /home/buildroot/buildroot/output/per-package/libbpf/host/bin/../lib/gcc/bpf-buildroot-none/12.1.0/include-fixed
End of search list.
GNU C17 (Buildroot 2022.05-118-ge052166011-dirty) version 12.1.0
(bpf-buildroot-none)
    compiled by GNU C version 12.1.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 9bf241ca1a2dd4ffd7652c5e247c9be8
minimal.bpf.c:6:1: error: expected '=', ',', ';', 'asm' or
'__attribute__' before '#pragma'
    6 | char LICENSE[] SEC("license") = "Dual BSD/GPL";
      | ^~~
minimal.bpf.c:6:1: error: expected identifier or '(' before '#pragma'
minimal.bpf.c:10:1: error: expected identifier or '(' before '#pragma'
   10 | SEC("tp/syscalls/sys_enter_write")
      | ^~~

So this is a bug (hard to call this a feature) in gcc (not even
bpf-gcc, I could repro with a simple gcc). Is there a bug reported for
this somewhere? Are GCC folks aware and working on the fix?

What's curious is that the only thing that allows to bypass this is
adding #x in macro, having #define DO_PRAGMA(x) _Pragma(x) doesn't
help.

So ideally GCC can fix this? But either way your patch as is
erroneously passing extra quoted strings to _Pragma().

I'm pondering whether it's just cleaner to define SEC() without
pragmas for GCC? It will only cause compiler warning about unnecessary
unused attribute for extern *variable* declarations, which are very
rare. Instead of relying on this quirky "fix" approach. Ideally,
though, GCC just fixes _Pragma() handling, of course.

quoted

quoted

 /* Avoid 'linux/stddef.h' definition of '__always_inline'. */
 #undef __always_inline
--
2.25.1