Re: [PATCH RFC] x86/lld: fix symbol map generation

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, May 03, 2022 at 10:17:44AM +0200, Jan Beulich wrote:
> On 02.05.2022 17:20, Roger Pau Monne wrote:
> > The symbol map generation (and thus the debug info attached to Xen) is
> > partially broken when using LLVM LD.  That's due to LLD converting
> > almost all symbols from global to local in the last linking step, and
> 
> I'm puzzled by "almost" - is there a pattern of which ones aren't
> converted?
> 
> Also "last linking step" is ambiguous, as we link three binaries and
> aiui the issue is present on every of these passes. May I suggest
> "... when linking actual executables" or (still somewhat ambiguous)
> "... when linking final binaries"?
> 
> > thus confusing tools/symbols into adding a file prefix to all text
> > symbols, the results looks like:
> > 
> > Xen call trace:
> >    [<ffff82d040449fe8>] R xxhash64.c#__start_xen+0x3938/0x39c0
> >    [<ffff82d040203734>] F __high_start+0x94/0xa0
> > 
> > In order to workaround this create a list of global symbols prior to
> > the linking step, and use objcopy to convert the symbols in the final
> > binary back to global before processing with tools/symbols.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > I haven't found a way to prevent LLD from converting the symbols, so
> > I've come up with this rather crappy workaround.
> 
> Perhaps a map file (like we use for shared libraries in tools/) would
> allow doing so? But of course this would want to be machine-generated,
> not manually maintained.
> 
> Have you gained any insight into _why_ they are doing what they do?

So after raising this in the LLVM LD forum, I was told this behavior
is due to the spec:

"A hidden symbol contained in a relocatable object must be either
removed or converted to STB_LOCAL binding by the link-editor when the
relocatable object is included in an executable file or shared
object."

Then I did some search myself and found that you raised the same with
GNU ld not doing the conversion:

https://sourceware.org/bugzilla/show_bug.cgi?id=12374

So it seems LLVM LD goes a bit further than GNU ld and also changes
the binding of symbols in the .symtab.  I'm not sure I would consider
the behavior of either linkers wrong.

As a test I've attempted to disable the hidden visibility setting we
set in compiler.h, just to realize that parts of our code do rely on
having hidden visibility.  That's the bug and alternative constructs
that use the "i" asm constrain with function pointers.  That's only
possible in the absence of a GOT or PLT table:

https://godbolt.org/z/jK3bq4fhe

So I think the way to fix this would be to set the visibility to
protected instead of hidden, and then to also make the setting of the
visibility unconditional: the compiler not supporting -fvisibility and
Xen not setting it will just lead to compiler errors further on during
the build process.

Thanks, Roger.