Commit `c841f52fcb`

Motiejus Jakštys <motiejus@uber.com>

2022-12-16 08:29:09

Elf: switch link order of libcompiler_rt and libc

Given `main.go`: package main import _ "os/user" func main() {} Compiling it to linux/arm64: $ CGO_CFLAGS='-O0' GOOS=linux GOARCH=arm64 CGO_ENABLED=1 CC="zig cc -target aarch64-linux-gnu.2.28" go build main.go Results in this error: runtime/cgo(.text): unknown symbol memset in callarm64 runtime/cgo(.text): unknown symbol memset in callarm64 runtime/cgo(.text): relocation target memset not defined In the midst of intermediate compilations files we can see this commmand: ld.lld -o _cgo_.o <...> /tmp/go-build206961058/b043/_x009.o <...> ~/.cache/zig/.../libcompiler_rt.a <...> ~/.cache/.../libc.so.6 `_x009.o` needs memset: $ readelf -Ws ./b043/_x009.o | grep memset 22: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND memset Both `libcompiler_rt.a` and `libc.so.6` provide it: $ readelf -Ws ~/.cache/zig/.../libcompiler_rt.a | grep memset 870: 0000000000000000 318 FUNC WEAK DEFAULT 519 memset $ readelf -Ws ~/.cache/zig/.../libc.so.6 | grep -w memset 476: 000000000001d34c 0 FUNC GLOBAL DEFAULT 7 memset@@GLIBC_2.2.5 Since `libcompiler_rt.a` comes before libc in the linker line, the resulting `_cgo_.o` still links to a weak, unversioned memset: $ readelf -Ws ./b043/_cgo_.o | grep -w memset 40: 000000000022c07c 160 FUNC WEAK DEFAULT 14 memset 719: 000000000022c07c 160 FUNC WEAK DEFAULT 14 memset Since the final linking step is done by Golang's linker, it does not know of `libcompiler_rt.a`, and fails to link with the error message above. However, Go linker does recognize memset from glibc. If we specify an `-lc` equivalent before the `libcompiler_rt.a`, it will link to memset from libc: $ readelf -Wa ./b043/_x009.o |grep memset 14: 0000000000000000 0 FUNC GLOBAL DEFAULT UND memset@GLIBC_2.17 (2) 157: 0000000000000000 0 FUNC GLOBAL DEFAULT UND memset@GLIBC_2.17 ... and then `main.go` will compile+link successfully. Why doesn't Go linker take memset from glibc? An educated guess: Go determines whether to link with glibc from what the program asks (I presume `.dynsym`). Since `memset` is no longer attributed to glibc, Go skips linking to glibc altogether. Bonus question: curious why `-O0` is necessary? Because when optimizations are enabled (the default), the C compiler replaces `memset` function call with plain `stp` instructions (on aarch64).

1 parent bcc2fb6

Changed files (1)

src

link

Elf.zig

@@ -1698,11 +1698,6 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
             try argv.append(ssp.full_object_path);
         }
 
-        // compiler-rt
-        if (compiler_rt_path) |p| {
-            try argv.append(p);
-        }
-
         // Shared libraries.
         if (is_exe_or_dyn_lib) {
             const system_libs = self.base.options.system_libs.keys();
@@ -1781,6 +1776,13 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
             }
         }
 
+        // compiler-rt. Since compiler_rt exports symbols like `memset`, it needs
+        // to be after the shared libraries, so they are picked up from the shared
+        // libraries, not libcompiler_rt.
+        if (compiler_rt_path) |p| {
+            try argv.append(p);
+        }
+
         // crt postlude
         if (csu.crtend) |v| try argv.append(v);
         if (csu.crtn) |v| try argv.append(v);

Commit c841f52fcb

Commit `c841f52fcb`