[Bug ld/26551] New: A definition referenced by an unneeded (--as-needed) shared object should be exported

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=26551

            Bug ID: 26551
           Summary: A definition referenced by an unneeded (--as-needed)
                    shared object should be exported
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

echo '.global _start; _start: ret' | as -o a.o
echo 'call _start' | as -o b.o
ld.bfd -shared b.o -o b.so

ld.bfd a.o b.so -o bfd.needed
ld.bfd a.o --as-needed b.so -o bfd.unneeded
ld.gold a.o b.so -o gold.needed
ld.gold a.o --as-needed b.so -o gold.unneeded
ld.lld a.o b.so -o lld.needed
ld.lld a.o --as-needed b.so -o lld.unneeded

bfd.unneeded does not export _start. All other 5 executables export _start.

_start should be exported. In a larger application, b.so may be referenced
through a shared object chain. Linking the executable against b.so makes the
intention clear that the definitions in regular objects resolving shared object
references are needed.

----

Here is a larger example related to --allow-shlib-undefined, demonstrating why
the GNU ld behavior (a definition referenced by an unneeded (--as-needed)
shared object is not exported) is not good. This may be related to bug 18652.


echo '.globl _start, myexit; _start: jmp foo; myexit: movq $60, %rax; movq $42,
%rdi; syscall' | as -o a.o
echo '.globl foo; foo: jmp bar' | as -o b.o
echo '.globl bar; bar: jmp myexit' | as -o c.o
ld.bfd -shared c.o -o c.so
ld.bfd -shared b.o ./c.so -o b.so
ld.bfd --dynamic-linker /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 a.o
/usr/lib/x86_64-linux-gnu/libc.so --as-needed ./b.so ./c.so -y myexit
--allow-shlib-undefined -o bfd.bad
ld.bfd --dynamic-linker /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 a.o
/usr/lib/x86_64-linux-gnu/libc.so --as-needed ./b.so ./c.so -y myexit -o
bfd.good

c.so is an "unneeded" (in the sense of --as-needed) shared object. In the
--allow-shlib-undefined command line, myexit is somehow not exported.

bfd.good exits with code 42.

% ./bfd.bad
./bfd.bad: symbol lookup error: ./c.so: undefined symbol: myexit

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