Re: [PATCH] force LTO to be disabled when compiling dlstart.lo

[Rich Felker <dalias@libc.org>]

On Tue, Apr 28, 2015 at 11:35:18AM +0300, Alexander Monakov wrote:
> Hello,
> 
> Sorry for not joining the discussion earlier.
> 
> Andre, can you specify your GCC and Binutils version?  The reason I ask, with
> modern toolchain you shouldn't be seeing the error you reported.  The fact
> that _dlstart_c function is used from assembly should have been communicated
> from the linker to the compiler via the "linker plugin".  If linker plugin was
> not used, that would explan the problem.
> 
> Can you also check if adding '-fuse-linker-plugin' to '-flto' works for you?
> 
> For reference, with GCC 4.9 that uses linker plugin for LTO automatically, I
> get the following diagnostics:
> 
> /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/../../../../x86_64-pc-linux-gnu/bin/ld:
> error: /tmp/ccxxkwJ8.ltrans0.ltrans.o: requires dynamic R_X86_64_PC32 reloc
> against '_dlstart_c' which may overflow at runtime; recompile with -fPIC
> /tmp/ccxxkwJ8.ltrans0.ltrans.o(.text+0x12): error: undefined reference to
> '_dlstart_c'
> 
> Not saying the patch can't go in -- just want to make sure everyone on the
> same page regarding the origin of the problem and GCC LTO capabilities.

This seems to be a common problem then. I helped someone on #gcc with
almost the exact same issue doing freestanding work making a
kernel/bare-metal app using LTO a week or so ago. I'm not sure the
linker plugin can solve the problem since it seems to happen for
symbol references *within* a single translation unit (or a combined .o
file produced by ld -r, as in the case of the person on #gcc) which
the linker plugin probably does not track.

Even if the problem is missing linker plugin though, I think we want
to avoid LTO on these files. It's likely to be very risky since the
code is running in a situation where no function calls, global data
accesses, or symbolic references are possible. Here we really are
asking the compiler to produce asm for us, rather than asking it to
produce an optimized way to get an abstract job done.

Rich