[vlc-devel] [PATCH 1/2] configure: allow toggling of -Wl, -z, defs and -no-undefined

[Thomas Guillem]

On Wed, Mar 15, 2017, at 13:51, Rémi Denis-Courmont wrote:
> On March 15, 2017 11:10:27 AM GMT+02:00, "Filip Roséen" <filip at atch.se>
> wrote:
> >There are cases where it is annoying to build VLC with this
> >restriction as you would like to use symbols that are simply not
> >present (in terms of definition) when things are being built.
> >
> >SUMMARY:
> >
> > - "-Wl,-z,defs" is now opt-out + automatic detection
> > - "-no-undefined" is now opt-out
> >
> >Both options are governed by a new configure flag,
> >"--{enable,disable}-no-undefined-symbols" as they are tightly coupled.
> >
> >--
> >
> >RATIONALE:
> >
> > - simplify usage of -fsanitize together with llvm/clang
> > - out-of-tree definitions of VLC related functions (I sometimes do
> >   this for extensive testing by building object files that I later
> >   circle between when trying out samples).
> >
> >Using llvm/clang with -fsanitize=address, the symbols put in the
> >resulting object-file related to the address sanitizer are not
> >available, and we will get an error diagnostic due to the relevant
> >flags.
> >
> >The below linked articles elaborates the issue further:
> >
> > - https://clang.llvm.org/docs/AddressSanitizer.html#usage
> > - https://github.com/google/sanitizers/wiki/AddressSanitizer#faq
> >
> >One can circumvent the issue by manually specifying flags to
> >effectivelly have the symbols resolved during linking, and preventing
> >use of the relevant shared library. The drawbacks are, of course:
> >
> > - negative impact on build time
> > - negative impact on execution speed
> > - unnecessary complex for those who are not familiar with the
> >   intrinsics of libtools and configure.ac
> >---
> > configure.ac | 24 +++++++++++++++---------
> > 1 file changed, 15 insertions(+), 9 deletions(-)
> >
> >diff --git a/configure.ac b/configure.ac
> >index 71f19c43f9..1081e14ec8 100644
> >--- a/configure.ac
> >+++ b/configure.ac
> >@@ -1086,16 +1086,22 @@ VLC_RESTORE_FLAGS
> > SYMBOLIC_LDFLAGS="${ac_cv_ld_bsymbolic}"
> > AC_SUBST(SYMBOLIC_LDFLAGS)
> > 
> >-VLC_SAVE_FLAGS
> >-LDFLAGS="${LDFLAGS} -Wl,-z,defs"
> >-AC_CACHE_CHECK([if linker supports -z,defs], [ac_cv_ld_z_defs], [
> >-  AC_TRY_LINK([],, [
> >-    ac_cv_ld_z_defs="yes"
> >-  ], [
> >-    ac_cv_ld_z_defs="no"
> >-  ])
> >+dnl
> >+dnl No undefined symbols for libvlc* objects
> >+dnl
> >+AC_ARG_ENABLE(no-undefined-symbols,
> >+  [AS_HELP_STRING([--disable-no-undefined-symbols],
> >+    [disable error on undefined symbols when linking \
> >+     core VLC objects (default enabled)] )] )
> >+
> >+AS_IF([test "${enable_no_undefined_symbols}" != "no" ], [
> >+  VLC_SAVE_FLAGS
> >+  LDFLAGS="${LDFLAGS} -Wl,-z,defs"
> >+  AC_CACHE_CHECK([if linker supports -z,defs], [ac_cv_ld_z_defs], [
> >+    AC_TRY_LINK([],, [ac_cv_ld_z_defs="yes"], [ac_cv_ld_z_defs="no"])
> >])
> >+  AS_IF([test "${ac_cv_ld_z_defs}" = "no"], [VLC_RESTORE_FLAGS])
> >+  AC_SUBST([LDFLAGS_no_undefined], [-no-undefined])
> > ])
> >-AS_IF([test "${ac_cv_ld_z_defs}" = "no"], [VLC_RESTORE_FLAGS])
> > 
> > dnl Check for __attribute__((packed))
> > AC_CACHE_CHECK([for __attribute__((packed))],
> >-- 
> >2.12.0
> >_______________________________________________
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> 
> I added this because I am fed up with devs not checking their
> dependencies correctly, breaking my and Debian builds frequently.
> 
> So obviously, I am very much against this change. This removes a very
> useful build-time diagnostic and WILL cause more build breaks.

This this won't be enabled by default. I guess devs won't enable it
unless they have to use a sanitizer. As sanitizer builds are very slow,
devs will keep a normal build with this diagnostic enabled (by default).

Personally, I develop mainly on a build with --enable-debug and without
any other options. When I want to use a sanitizer, a use an other dir
with different configure rules.

> -- 
> Rémi Denis-Courmont
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