Message-ID: <CAEf4BzZkSXLqFz4Cjx4_Z_0sxBBSd-SEhT8u+3EZVccqH7qXkg@mail.gmail.com>
Date:   Thu, 30 Jun 2022 14:51:43 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     James Hilliard <james.hilliard1@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] libbpf: fix broken gcc SEC pragma macro

On Mon, Jun 27, 2022 at 9:43 PM James Hilliard
<james.hilliard1@...il.com> wrote:
>
> On Mon, Jun 27, 2022 at 5:16 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Thu, Jun 9, 2022 at 4:27 PM James Hilliard <james.hilliard1@...il.com> wrote:
> > >
> > > On Thu, Jun 9, 2022 at 12:13 PM Andrii Nakryiko
> > > <andrii.nakryiko@...il.com> wrote:
> > > >
> > > > On Wed, Jun 8, 2022 at 11:24 PM James Hilliard
> > > > <james.hilliard1@...il.com> wrote:
> > > > >
> > > > > 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 <james.hilliard1@...il.com>
> > > > > ---
> > > > > 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?
>
> Yeah, saw a few issues that looked relevant:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55578
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90400
>
> >
> > 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?
>
> From the reported issues...it doesn't sound like a fix is going to be
> coming all that
> soon in GCC.
>
> > But either way your patch as is
> > erroneously passing extra quoted strings to _Pragma().
>
> I recall the extra quotes were needed to make this work, does it work for you
> without them?
>
> >
> > 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.
>
> I mean, as long as this workaround is reliable I'd say using it is the
> best option
> for backwards compatibility, especially since it's only needed in one place from
> the looks of it.

Is it reliable, though? Adding those quotes breaks Clang (I checked)
and it doesn't work as expected with GCC as well. It stops complaining
about #pragma, but it also doesn't push -Wignored-attributes. Here's
the test:

#define DO_PRAGMA(x) _Pragma(#x)

#define SEC(name) \
       DO_PRAGMA("GCC diagnostic push")                                    \
       DO_PRAGMA("GCC diagnostic ignored \"-Wignored-attributes\"")        \
        __attribute__((section(name), used))                               \
       DO_PRAGMA("GCC diagnostic pop")                                     \

extern int something SEC("whatever");

int main()
{
        return something;
}


Used like this you get same warning:

$ cc test.c
test.c:10:1: warning: ‘used’ attribute ignored [-Wattributes]
   10 | extern int something SEC("whatever");
      | ^~~~~~

Removing quotes fixes Clang (linker error is expected)

$ clang test.c
/opt/rh/gcc-toolset-11/root/usr/lib/gcc/x86_64-redhat-linux/11/../../../../bin/ld:
/tmp/test-4eec0b.o: in function `main':
test.c:(.text+0xe): undefined reference to `something'

But we get back to the original problem with GCC:

$ cc test.c
test.c:10:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’
before ‘#pragma’
   10 | extern int something SEC("whatever");
      | ^~~
test.c:10:1: error: expected identifier or ‘(’ before ‘#pragma’
test.c: In function ‘main’:
test.c:14:16: error: ‘something’ undeclared (first use in this function)
   14 |         return something;
      |                ^~~~~~~~~


So the best way forward I can propose for you is this:


#if __GNUC__ && !__clang__

#define SEC(name) __attribute__((section(name), used))

#else

#define SEC(name) \
        _Pragma("GCC diagnostic push")                                      \
        _Pragma("GCC diagnostic ignored \"-Wignored-attributes\"")          \
        __attribute__((section(name), used))                                \
        _Pragma("GCC diagnostic pop")                                       \

#endif

extern int something SEC("whatever");

int main()
{
        return something;
}


With some comments explaining how broken GCC is w.r.t. _Pragma. And
just live with compiler warning about used if used with externs.


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

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