[PATCH] D89152: [lld][WebAssembly] Add support for -Bsymbolic

[Fangrui Song via Phabricator via llvm-commits]

MaskRay accepted this revision.
MaskRay added inline comments.


================
Comment at: lld/wasm/Driver.cpp:496
+  if (config->bsymbolic && (!config->pie && !config->shared)) {
+    warn("-Bsymbolic is only meaningful when combined with -shared, or -pie");
+  }
----------------
sbc100 wrote:
> MaskRay wrote:
> > MaskRay wrote:
> > > sbc100 wrote:
> > > > sbc100 wrote:
> > > > > MaskRay wrote:
> > > > > > In ELF, -Bsymbolic is redundant for executables (-no-pie or -pie).
> > > > > > 
> > > > > Interesting, so you are saying the `-Bsymbolic` is basically the default for executable?
> > > > > 
> > > > > Wouldn't that mean that, for example, LD_PRELOAD would not work for symbols in the main executable?    My understanding is that `-Bsymbolic` would prevent `LD_PRELOAD` from overriding symbols because they would be bound locally.  Although I guess I am wrong?
> > > > Indeed you are right, it looks like LD_PRELOAD as no effect on the main executable itself.  TIL!
> > > -Bsymbolic is basically the default for executable.
> > > The symbol resolution order: exe, preloaded, needed
> > > 
> > > In preloaded, the LD_PRELOADed library is listed in a breadth-first search order.
> > > needed is similar.
> > > 
> > > 
> > I was wrong about the symbol resolution order.
> > 
> > In musl, LD_PRELOAD libraries are prepended to the DT_NEEDED list. The resolution order is a breadth-first search order starting from the executable, i.e. the order may be:
> > `exe, preload0, preload1, needed0, needed1, needd0_of_preload0, needed1_of_preload0, needd0_of_needed0, ...`
> Thanks for the update.   The only thing that this change does now is to bind symbols in the executable (and `-Bsymbolic` libraries) to local definition (if they exist).   
> 
> We have no concept of LD_PRELOAD yet in the WebAssembly (or emscripten) world.
> 
> Any objections to landing this change?
No objection (just wanted to mention that in ELF linkers, they don't warn about this combo).

The nice thing with the ELF behavior is that: if a project deliberately uses -Bsymbolic, it does not need to differentiate executable linker options from shared object linker options.

I know really little about wasm, though.


Repository:
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  https://reviews.llvm.org/D89152/new/

https://reviews.llvm.org/D89152