LLD doesn't implement --rpath-link; doesn't error for an versioned symbol error from a transitive DSO

[Kartatz]

Steps to reproduce

Step 1

Create a versioned libfoo:

echo 'void foo(void) {}' > foo.c

cat << code > 'libfoo.ver'
VERSION_1.0 {
    global:
        foo;
    local:
        *;
};
code

gcc foo.c -shared -o libfoo.so -Wl,--version-script=libfoo.ver

Step 2

Now create a library that depends on libfoo (libbar):

cat << code > bar.c
void foo(void);                                      
void bar(void) { foo(); }
code

gcc bar.c -shared -o libbar.so -lfoo -L.

Step 3

Finally, create an executable that depends on libbar:

cat << code > main.c
void bar(void);
int main(void) { bar(); }
code

Step 4

Confirm that linking with both BFD and LLD works:

$ gcc main.c -o main -lbar -L. -fuse-ld=bfd
<succeeds>
$ gcc main.c -o main -lbar -L. -fuse-ld=lld
<succeeds>

Step 5

Now recompile libfoo, this time without symbol versioning:

$ gcc foo.c -shared -o libfoo.so

Linking with BFD now reports an undefined symbol (expected behavior):

$ gcc main.c -o main -lbar -L. -fuse-ld=bfd
ld: ./libbar.so: undefined reference to `foo@VERSION_1.0'
collect2: error: ld returned 1 exit status

LLD, however, links without reporting any errors (unexpected behavior):

$ gcc main.c -o main -lbar -L. -fuse-ld=lld
<succeeds>

LLD version

$ ld.lld --version
LLD 21.1.0 (compatible with GNU linkers)

[MaskRay]

This is an intended behavior. While GNU ld traces all shared objects (transitive DT_NEEDED dependencies) and emulates the behavior of a dynamic loader to warn more cases, other linkers, including lld, don't implement this rule.

https://maskray.me/blog/2020-12-19-lld-and-gnu-linker-incompatibilities --no-allow-shlib-undefined and --rpath-link

gold and ld.lld implement a simplified version. They warn for shared objects whose DT_NEEDED dependencies are all seen as input files.
ld.lld accepts and ignores --rpath-link

---

Step 4

Confirm that linking with both BFD and LLD works:

The BFD link actually fails

% gcc main.c -o main -lbar -L. -fuse-ld=bfd
/usr/bin/ld.bfd: warning: libfoo.so, needed by ./libbar.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld.bfd: ./libbar.so: undefined reference to `foo@VERSION_1.0'
collect2: error: ld returned 1 exit status
% gcc main.c -o main -lbar -L. -fuse-ld=bfd -Wl,-rpath-link=.

At step 5, you will get an error if -lfoo is also specified

% gcc main.c -o main -lbar -L. -fuse-ld=lld -lfoo
ld.lld: error: undefined reference: foo@VERSION_1.0
>>> referenced by ./libbar.so (disallowed by --no-allow-shlib-undefined)
collect2: error: ld returned 1 exit status