Re: [PATCH] libgcc: make heap-based trampolines conditional on libc presence

[Sergei Trofimovich <slyich@gmail.com>]

On Mon, 23 Oct 2023 13:54:01 +0100
Iain Sandoe <iain@sandoe.co.uk> wrote:

> hi Sergei,
> 
> > On 23 Oct 2023, at 13:43, Sergei Trofimovich <slyich@gmail.com> wrote:
> > 
> > From: Sergei Trofimovich <siarheit@google.com>
> > 
> > To build `libc` for a target one needs to build `gcc` without `libc`
> > support first. Commit r14-4823-g8abddb187b3348 "libgcc: support
> > heap-based trampolines" added unconditional `libc` dependency and broke
> > libc-less `gcc` builds.
> > 
> > An example failure on `x86_64-unknown-linux-gnu`:
> > 
> >    $ mkdir -p /tmp/empty
> >    $ ../gcc/configure \
> >        --disable-multilib \
> >        --without-headers \
> >        --with-newlib \
> >        --enable-languages=c \
> >        --disable-bootstrap \
> >        --disable-gcov \
> >        --disable-threads \
> >        --disable-shared \
> >        --disable-libssp \
> >        --disable-libquadmath \
> >        --disable-libgomp \
> >        --disable-libatomic \
> >        --with-build-sysroot=/tmp/empty
> >    $ make
> >    ...
> >    /tmp/gb/./gcc/xgcc -B/tmp/gb/./gcc/ -B/usr/local/x86_64-pc-linux-gnu/bin/ -B/usr/local/x86_64-pc-linux-gnu/lib/ -isystem /usr/local/x86_64-pc-linux-gnu/include -isystem /usr/local/x86_64-pc-linux-gnu/sys-include --sysroot=/tmp/empty   -g -O2 -O2  -g -O2 -DIN_GCC   -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition  -isystem ./include  -fpic -mlong-double-80 -DUSE_ELF_SYMVER -fcf-protection -mshstk -g -DIN_LIBGCC2 -fbuilding-libgcc -fno-stack-protector -Dinhibit_libc -fpic -mlong-double-80 -DUSE_ELF_SYMVER -fcf-protection -mshstk -I. -I. -I../.././gcc -I/home/slyfox/dev/git/gcc/libgcc -I/home/slyfox/dev/git/gcc/libgcc/. -I/home/slyfox/dev/git/gcc/libgcc/../gcc -I/home/slyfox/dev/git/gcc/libgcc/../include  -DHAVE_CC_TLS  -DUSE_TLS  -o heap-trampoline.o -MT heap-trampoline.o -MD -MP -MF heap-trampoline.dep  -c .../gcc/libgcc/config/i386/heap-trampoline.c -fvisibility=hidden -DHIDE_EXPORTS
> >    ../gcc/libgcc/config/i386/heap-trampoline.c:3:10: fatal error: unistd.h: No such file or directory
> >        3 | #include <unistd.h>
> >          |          ^~~~~~~~~~
> >    compilation terminated.
> >    make[2]: *** [.../gcc/libgcc/static-object.mk:17: heap-trampoline.o] Error 1
> >    make[2]: Leaving directory '/tmp/gb/x86_64-pc-linux-gnu/libgcc'
> >    make[1]: *** [Makefile:13307: all-target-libgcc] Error 2
> > 
> > The change inhibits any heap-based trampoline code.  
> 
> That looks reasonable to me (I was considering using __has_include(), but the inhibit_libc is neater).
> 
> The fact that this first compiler is buit without heap-trampoline support, would become relevant, I guess if libc wanted to use them, it would need another iteration.
> 
> so, it looks fine, but I cannot actually approve it.

Sounds good. Let's wait for others to chime in. Maybe Richard? :)

AFAIU libcs (like `glibc`) try hard not to use link tests and uses
mainly preprocessor and code generator to specifically accommodate this
case. Maybe there is a way to pass the support flag to libc without the
reliance on code presence in libgcc.

Otherwise we could use __builtin_trap() as an implementation for exposed
symbols.

> 
> > 
> > libgcc/
> > 
> > 	* libgcc/config/aarch64/heap-trampoline.c: Disable when libc is
> > 	  not present.
> > ---
> > libgcc/config/aarch64/heap-trampoline.c | 5 +++++
> > libgcc/config/i386/heap-trampoline.c    | 5 +++++
> > 2 files changed, 10 insertions(+)
> > 
> > diff --git a/libgcc/config/aarch64/heap-trampoline.c b/libgcc/config/aarch64/heap-trampoline.c
> > index c8b83681ed7..f22233987ca 100644
> > --- a/libgcc/config/aarch64/heap-trampoline.c
> > +++ b/libgcc/config/aarch64/heap-trampoline.c
> > @@ -1,5 +1,8 @@
> > /* Copyright The GNU Toolchain Authors. */
> > 
> > +/* libc is required to allocate trampolines.  */
> > +#ifndef inhibit_libc
> > +
> > #include <unistd.h>
> > #include <sys/mman.h>
> > #include <stdint.h>
> > @@ -170,3 +173,5 @@ __builtin_nested_func_ptr_deleted (void)
> >       tramp_ctrl_curr = prev;
> >     }
> > }
> > +
> > +#endif /* !inhibit_libc */
> > diff --git a/libgcc/config/i386/heap-trampoline.c b/libgcc/config/i386/heap-trampoline.c
> > index 96e13bf828e..4b9f4365868 100644
> > --- a/libgcc/config/i386/heap-trampoline.c
> > +++ b/libgcc/config/i386/heap-trampoline.c
> > @@ -1,5 +1,8 @@
> > /* Copyright The GNU Toolchain Authors. */
> > 
> > +/* libc is required to allocate trampolines.  */
> > +#ifndef inhibit_libc
> > +
> > #include <unistd.h>
> > #include <sys/mman.h>
> > #include <stdint.h>
> > @@ -170,3 +173,5 @@ __builtin_nested_func_ptr_deleted (void)
> >       tramp_ctrl_curr = prev;
> >     }
> > }
> > +
> > +#endif /* !inhibit_libc */
> > -- 
> > 2.42.0
> >   
> 


-- 

  Sergei