PoC weak symbol handling

[mati865]

Proof of concept for #1817 (comment)

[davidlattimore]

I tried the following test with this change:

diff --git a/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def-1.c b/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def-1.c
new file mode 100644
index 0000000000..6cf1236844
--- /dev/null
+++ b/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def-1.c
@@ -0,0 +1,7 @@
+int foo(void) {
+    return 42;
+}
+
+int bar(void) {
+    return 42;
+}
diff --git a/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def-2.c b/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def-2.c
new file mode 100644
index 0000000000..8737aa7886
--- /dev/null
+++ b/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def-2.c
@@ -0,0 +1,5 @@
+int bar(void);
+
+int call_bar(void) {
+    return bar();
+}
diff --git a/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def.c b/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def.c
new file mode 100644
index 0000000000..3a1d90ba6e
--- /dev/null
+++ b/wild/tests/sources/elf/weak-ref-strong-def/weak-ref-strong-def.c
@@ -0,0 +1,31 @@
+//#Config:shared
+//#Mode:dynamic
+//#Object:runtime.c
+//#Shared:weak-ref-strong-def-1.c
+//#Object:weak-ref-strong-def-2.c
+//#ExpectDynSym:foo binding=weak
+//#ExpectDynSym:bar binding=global
+//#DiffIgnore:section.got
+//#DiffIgnore:.dynamic.DT_NEEDED
+
+#include "../common/runtime.h"
+
+// This weak reference is the only reference to foo.
+int __attribute__((weak)) foo(void);
+
+// For bar, there's also a non-weak reference in a different object file.
+int __attribute__((weak)) bar(void);
+
+void _start(void) {
+  runtime_init();
+
+  if (!foo || foo() != 42) {
+    exit_syscall(10);
+  }
+
+  if (!bar || bar() != 42) {
+    exit_syscall(11);
+  }
+
+  exit_syscall(42);
+}

Linker-diff showed diffs for bar being weak in wild's output but non-weak in GNU ld's output.

[mati865]

There is also a race condition there. Once in a few runs this passes:

❯ nt elf/x86_64/weak-ref-strong-def/shared
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.05s
────────────
 Nextest run ID 364bcfff-45a1-4d5d-b132-36470e95a3ac with nextest profile: default
    Starting 1 test across 9 binaries (889 tests skipped)
        PASS [   0.010s] wild-linker::integration_tests elf/x86_64/weak-ref-strong-def/shared
────────────
     Summary [   0.011s] 1 test run: 1 passed, 889 skipped

❯ readelf -WDs /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def.c.wild

Symbol table for image contains 3 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND bar
     2: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND foo

While failing case produces:

❯ readelf -WDs /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def.c.wild

Symbol table for image contains 3 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND bar
     2: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND foo

Also, sym-info:

Details

Weak bar (failing test)

lobal name `bar` refers to: Some(sym-38)
Definitions / references with name `bar`:
  31 -> 38: Weak Undefined: ABSOLUTE | WEAK
    bar
    #8 in File #256 (1/0) /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def.c.o (LOADED)
  38: Global Func: DYNAMIC | FUNCTION | GOT | PLT | WEAK
    bar
    #1 in File #768 (3/0) /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def-1.c.wild.so (LOADED)
  44 -> 38: Global Undefined: ABSOLUTE
    bar
    #4 in File #1024 (4/0) /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def-2.c.o (LOADED)

Strong bar (passing test):

Global name `bar` refers to: Some(sym-38)
Definitions / references with name `bar`:
  31 -> 38: Weak Undefined: ABSOLUTE | WEAK
    bar
    #8 in File #256 (1/0) /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def.c.o (LOADED)
  38: Global Func: DYNAMIC | FUNCTION | GOT | PLT
    bar
    #1 in File #768 (3/0) /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def-1.c.wild.so (LOADED)
  44 -> 38: Global Undefined: ABSOLUTE
    bar
    #4 in File #1024 (4/0) /home/mateusz/Projects/wild/wild/tests/build/elf/x86_64/weak-ref-strong-def/shared/weak-ref-strong-def-2.c.o (LOADED)

I guess once in a while we process 31 before 44, so 31 adds WEAK and 44 removes it. I had posted this PoC because I was wondering if this is somewhat close to being a solution. Thank you for showing me how well it falls apart 😆.