`lld` does not emit `PT_INTERP` when using `--dynamic-linker` with `-shared` (works with GNU ld/bfd)

[izissise]

Description

When linking with lld and passing both -shared and --dynamic-linker=/lib64/ld-linux-x86-64.so.2, the resulting ELF shared object does not contain a PT_INTERP program header.
This differs from GNU ld (bfd), which does emit a PT_INTERP in the same situation.

This causes executables built as cdylib+-shared+--dynamic-linker to segfault when run directly, because the runtime loader is never invoked.

If run explicitly with the loader (/lib64/ld-linux-x86-64.so.2 ./libmylib.so), the binary executes correctly.

Steps to Reproduce

1. Minimal C file:

   int main() { return 42; }

2. Link with lld:

   clang -fuse-ld=lld -shared test.c \
     /usr/lib/x86_64-linux-gnu/Scrt1.o \
     -Wl,--dynamic-linker=/lib64/ld-linux-x86-64.so.2 \
     -o test.so

3. Inspect program headers:

   readelf -l test.so | grep INTERP

   → no INTERP segment is present.

4. Run:

   ./test.so

   → segfault (jump to 0).

5. Run with loader explicitly:

   /lib64/ld-linux-x86-64.so.2 ./test.so

   → works, returns exit code 42.

6. Now repeat with GNU ld (bfd):

   clang -fuse-ld=bfd -shared test.c \
     /usr/lib/x86_64-linux-gnu/Scrt1.o \
     -Wl,-pie \
     -Wl,--dynamic-linker=/lib64/ld-linux-x86-64.so.2 \
     -o test_bfd.so
   readelf -l test_bfd.so | grep INTERP

   → INTERP is present, and ./test_bfd.so runs fine.

Analysis

Looking at lld source:

• needsInterpSection in lld/ELF/SyntheticSections.cpp:

  static bool needsInterpSection(Ctx &ctx) {
    return !ctx.arg.relocatable && !ctx.arg.shared &&
           !ctx.arg.dynamicLinker.empty() && ctx.script->needsInterpSection();
  }

  → Explicitly excludes the .interp section when -shared is passed.

• Writer<ELFT>::createPhdrs in lld/ELF/Writer.cpp:

  if (OutputSection *cmd = findSection(ctx, ".interp", partNo))
      addHdr(PT_INTERP, cmd->getPhdrFlags())->add(cmd);

  → Since .interp is never created in -shared mode, PT_INTERP never gets emitted.

This is the direct cause: lld refuses to emit PT_INTERP for -shared binaries, even if --dynamic-linker is given.

Expected Behavior

When passing -shared and --dynamic-linker=..., lld should:

• Emit .interp section
• Emit a PT_INTERP program header
  so the shared object is executable directly via the kernel, matching GNU ld (bfd).

Actual Behavior

• .interp and PT_INTERP are not emitted by lld.
• Running the .so directly segfaults, because the loader is never invoked.

Impact

• Rust issue for reference: rust-lang/rust#146780

Environment

• Rust 1.90.0 stable (where lld became default for x86_64-unknown-linux-gnu)
• LLVM/lld master (current behavior still the same)
• Reproduced on Ubuntu 22.04 (glibc-based)

Suggested Fix

Loosen the condition in needsInterpSection() so that .interp can be emitted if --dynamic-linker is provided, even under -shared. This matches bfd behavior and would fix the segfault.

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: Hugues Morisset (izissise)

### Description

When linking with lld and passing both -shared and --dynamic-linker=/lib64/ld-linux-x86-64.so.2, the resulting ELF shared object does not contain a PT_INTERP program header.
This differs from GNU ld (bfd), which does emit a PT_INTERP in the same situation.

This causes executables built as cdylib+-shared+--dynamic-linker to segfault when run directly, because the runtime loader is never invoked.

If run explicitly with the loader (/lib64/ld-linux-x86-64.so.2 ./libmylib.so), the binary executes correctly.

Steps to Reproduce

1. Minimal C file:

   int main() { return 42; }

2. Link with lld:

   clang -fuse-ld=lld -shared test.c \
     /usr/lib/x86_64-linux-gnu/Scrt1.o \
     -Wl,--dynamic-linker=/lib64/ld-linux-x86-64.so.2 \
     -o test.so

3. Inspect program headers:

   readelf -l test.so | grep INTERP

   → no INTERP segment is present.

4. Run:

   ./test.so

   → segfault (jump to 0).

5. Run with loader explicitly:

   /lib64/ld-linux-x86-64.so.2 ./test.so

   → works, returns exit code 42.

6. Now repeat with GNU ld (bfd):

   clang -fuse-ld=bfd -shared test.c \
     /usr/lib/x86_64-linux-gnu/Scrt1.o \
     -Wl,-pie \
     -Wl,--dynamic-linker=/lib64/ld-linux-x86-64.so.2 \
     -o test_bfd.so
   readelf -l test_bfd.so | grep INTERP

   → INTERP is present, and ./test_bfd.so runs fine.

Analysis

Looking at lld source:

• needsInterpSection in lld/ELF/SyntheticSections.cpp:

  static bool needsInterpSection(Ctx &ctx) {
    return !ctx.arg.relocatable && !ctx.arg.shared &&
           !ctx.arg.dynamicLinker.empty() && ctx.script->needsInterpSection();
  }

  → Explicitly excludes the .interp section when -shared is passed.

• Writer<ELFT>::createPhdrs in lld/ELF/Writer.cpp:

  if (OutputSection *cmd = findSection(ctx, ".interp", partNo))
      addHdr(PT_INTERP, cmd->getPhdrFlags())->add(cmd);

  → Since .interp is never created in -shared mode, PT_INTERP never gets emitted.

This is the direct cause: lld refuses to emit PT_INTERP for -shared binaries, even if --dynamic-linker is given.

Expected Behavior

When passing -shared and --dynamic-linker=..., lld should:

• Emit .interp section
• Emit a PT_INTERP program header
  so the shared object is executable directly via the kernel, matching GNU ld (bfd).

Actual Behavior

• .interp and PT_INTERP are not emitted by lld.
• Running the .so directly segfaults, because the loader is never invoked.

Impact

• Rust issue for reference: rust-lang/rust#146780

Environment

• Rust 1.90.0 stable (where lld became default for x86_64-unknown-linux-gnu)
• LLVM/lld master (current behavior still the same)
• Reproduced on Ubuntu 22.04 (glibc-based)

Suggested Fix

Loosen the condition in needsInterpSection() so that .interp can be emitted if --dynamic-linker is provided, even under -shared. This matches bfd behavior and would fix the segfault.

[smithp35]

The ELF specification only defines a PT_INTERP program header for executables https://gabi.xinuos.com/elf/08-dynamic.html . While I acknowledge the difference in behaviour between lld and GNU ld, it looks like this is a very niche use case. As of now I suspect that this won't be a priority for existing contributors. You may want to give this a go yourself with a PR.

As a workaround you may be able achieve what you want with a linker script and the PHDRS command https://sourceware.org/binutils/docs/ld/PHDRS.html

You'll need to create a .interp section containing the string with your dynamic loader, then assign that section to the PT_INTERP program header. There is an example at the bottom of the binutils docs.

Code for program headers in presence of PHDRS
https://github.com/llvm/llvm-project/blob/main/lld/ELF/LinkerScript.cpp#L1750

[izissise]

Thanks a lot for the links to workarounds, I might try to implement the behavior myself no promises.

I think it should at least give an error that -Wl,--dynamic-linker is not supported with -shared

[izissise]

I can confirm that relaxing the condition for needsInterpSection fixes it

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index 457a794a8c3a..040911a8b505 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -4543,7 +4543,7 @@ void PartitionIndexSection::writeTo(uint8_t *buf) {
 }
 
 static bool needsInterpSection(Ctx &ctx) {
-  return !ctx.arg.relocatable && !ctx.arg.shared &&
+  return !ctx.arg.relocatable &&
          !ctx.arg.dynamicLinker.empty() && ctx.script->needsInterpSection();
 }
 

I'll look into adding a unit tests and proposing a proper patch

[MaskRay]

This will break linking glibc with lld.

I think the statement about GNU ld behavior is incorrect.

The -Wl,-pie command line passes these options -shared -pie --dynamic-linker=x.so to ld.bfd. In GNU ld, -pie overrides -shared and --dynamic-linker= is allowed with -pie. GNU ld doesn't support --dynamic-linker= when producing a DSO.

lld rejects -pie -shared.

To create an executable file that can also be dlopened, specify -pie and drop -shared.

[frobtech]

Existing practice is to write an explicit .interp input section (in C or assembly) in an input file to a -shared link, rather than using -dynamic-linker.

[izissise]

Thanks for the pointers that's really helpful!