[PATCH] D30812: AsmPrinter: Don't treat symbols with prefix data as code

[Peter Collingbourne via Phabricator via llvm-commits]

pcc added a comment.

Does this really solve your problem? The linker should only create the trampoline if an object that takes a reference to the function with prefix data was built without `-fPIC`. If you change the symbol type to object I would expect the linker to create a copy relocation instead. That would cause the dynamic loader to create a copy of the function body in the main executable's address space, and that copy would not be in executable memory so you would not be able to call it.

In fact, if I patch in your change and compile your test case, that is exactly what happens:

  $ readelf -r main
  
  Relocation section '.rela.dyn' at offset 0x508 contains 2 entries:
    Offset          Info           Type           Sym. Value    Sym. Name + Addend
  000000600ff8  000400000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
  000000601040  000c00000005 R_X86_64_COPY     0000000000601040 hello + 0
  
  Relocation section '.rela.plt' at offset 0x538 contains 3 entries:
    Offset          Info           Type           Sym. Value    Sym. Name + Addend
  000000601018  000200000007 R_X86_64_JUMP_SLO 0000000000000000 printf + 0
  000000601020  000300000007 R_X86_64_JUMP_SLO 0000000000000000 __libc_start_main + 0
  000000601028  000400000007 R_X86_64_JUMP_SLO 0000000000000000 __gmon_start__ + 0

and if I change your test case like this:

  diff --git a/main.c b/main.c
  index d54d1c5..accf187 100644
  --- a/main.c
  +++ b/main.c
  @@ -5,5 +5,6 @@ int hello(void);
   int main() {
           int *prefix_data = (int*) &hello;
           printf("hi: %d\n", prefix_data[-1]);
  +        hello();
           return 0;
   }

I get:

  $ LD_LIBRARY_PATH=. ./main
  hi: 0
  Segmentation fault (core dumped)

I think the correct fix is for you to arrange to build any objects that may access prefix data with `-fPIC` (or with the `-relocation-model=pic` flag to `llc`). If I change your test case to do that:

  diff --git a/Makefile b/Makefile
  index 84ccf76..9d22147 100644
  --- a/Makefile
  +++ b/Makefile
  @@ -6,7 +6,7 @@ all : libtest.s main
   libtest.so : libtest.o
          gcc -shared -o $@ $+
   
  -main : main.o libtest.so
  +main : main.c libtest.so
          gcc -fPIC -L. -ltest -o $@ $+
   
   run : main

the `main` binary works as intended.


https://reviews.llvm.org/D30812