[libc] Add Scrt1.o to startup_components

[izaakschroeder]

Needed for -fpie targets. Don't know if this is safe or not... but it seems to run compiled C++ executables.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Izaak Schroeder (izaakschroeder)

Changes

Needs for -fpie targets. Don't know if this is safe or not... but it seems to run compiled executables.

---

Full diff: https://github.com/llvm/llvm-project/pull/97497.diff

1 Files Affected:

• (modified) libc/startup/linux/CMakeLists.txt (+10-1)

diff --git a/libc/startup/linux/CMakeLists.txt b/libc/startup/linux/CMakeLists.txt
index 336c5d0f6bfa2..b721d15091227 100644
--- a/libc/startup/linux/CMakeLists.txt
+++ b/libc/startup/linux/CMakeLists.txt
@@ -118,6 +118,15 @@ merge_relocatable_object(
   .do_start
 )
 
+# HACK: Is this even ok?
+# For reference: `Scrt1` is used when `-fpie` vs `crt1` for `-fno-pie`
+merge_relocatable_object(
+  Scrt1
+  .${LIBC_TARGET_ARCHITECTURE}.start
+  .${LIBC_TARGET_ARCHITECTURE}.tls
+  .do_start
+)
+
 add_startup_object(
   crti
   SRC
@@ -131,7 +140,7 @@ add_startup_object(
 )
 
 add_custom_target(libc-startup)
-set(startup_components crt1 crti crtn)
+set(startup_components Scrt1 crt1 crti crtn)
 foreach(target IN LISTS startup_components)
   set(fq_target_name libc.startup.linux.${target})
   add_dependencies(libc-startup ${fq_target_name})

[izaakschroeder]

/cc @michaelrj-google @jhuber6 @intue @SchrodingerZhu

[SchrodingerZhu]

cc @MaskRay . I am not sure directly creating crt1 to Scrt1 based on crt1 will work? If it does work for current configuration, it can be easily broken in the future. -pie, -static, -static-pie should have different relocation policies.

[izaakschroeder]

@SchrodingerZhu @MaskRay is there a simple path forward I can take, even if temporarily, to keep the ball moving? Like adding a .start-pie / .tls-pie (based on .start and .tls respectively) or something that is a copy but that could easily be modified in the future?

[MaskRay]

cc @MaskRay . I am not sure directly creating crt1 to Scrt1 based on crt1 will work? If it does work for current configuration, it can be easily broken in the future. -pie, -static, -static-pie should have different relocation policies.

I am not familiar with the build system, but you can create Scrt1.o and make crt1.o a symlink.
For -static-pie, rcrt1.o is needed and it needs to resolve relative relocations (also IRELATIVE if ifunc is supported).

[jhuber6]

cc @MaskRay . I am not sure directly creating crt1 to Scrt1 based on crt1 will work? If it does work for current configuration, it can be easily broken in the future. -pie, -static, -static-pie should have different relocation policies.

I am not familiar with the build system, but you can create Scrt1.o and make crt1.o a symlink. For -static-pie, rcrt1.o is needed and it needs to resolve relative relocations (also IRELATIVE if ifunc is supported).

Do you know where the differences between these files is documented?

[MaskRay]

cc @MaskRay . I am not sure directly creating crt1 to Scrt1 based on crt1 will work? If it does work for current configuration, it can be easily broken in the future. -pie, -static, -static-pie should have different relocation policies.

I am not familiar with the build system, but you can create Scrt1.o and make crt1.o a symlink. For -static-pie, rcrt1.o is needed and it needs to resolve relative relocations (also IRELATIVE if ifunc is supported).

Do you know where the differences between these files is documented?

They are undocumented. One needs to investigate whether other libc implementations do...

[jhuber6]

They are undocumented. One needs to investigate whether other libc implementations do...

Good to know my failing to find anything online wasn't a skill issue, thanks.

[nickdesaulniers]

Throwing a "minus one" on this for now until we better understand if/why this is necessary.