Fix PLT creation for i386

i386 target does not have PC relative loads. Its ABI therefore require
ebx register to points to the GOT when executing a PLT entry. This means
that PLT entry cannot be used transparently, the compiler needs to
expect execution of a PLT entry to be able to use one, that is a PLT
entry should only be created if the relocation explicitely asks for it
(eg. R_386_PLT32).

This patch creates a new target macro PCRELATIVE_DLLPLT to indicate
whether a target can do a PC relative load in PLT entry when building a
dynamic library. Executable do not normally pose a problem because they
are loaded at a fixed address and thus the absolute address of GOT can
be used.

Note that in such a case, if the compiler does not use a PLT aware
relocation for external access then the code relocation will fall on the
dynamic loader since there is no PLT entry to relocate too.

1 files changed, 11 insertions, 1 deletions

diff --git a/tccelf.c b/tccelf.c
index 7f1679d..df7375c 100644
--- a/tccelf.c
+++ b/tccelf.c
@@ -1052,7 +1052,8 @@ ST_FUNC void build_got_entries(TCCState *s1)
             /* Proceed with PLT/GOT [entry] creation if any of the following
                condition is met:
                - it is an undefined reference (dynamic relocation needed)
-               - symbol is absolute (probably created by tcc_add_symbol)
+               - symbol is absolute (probably created by tcc_add_symbol and
+                 thus might be too far from application code)
                - relocation requires a PLT/GOT (BUILD_GOTPLT_ENTRY or
                  ALWAYS_GOTPLT_ENTRY). */
             if (sym->st_shndx != SHN_UNDEF &&
@@ -1060,6 +1061,15 @@ ST_FUNC void build_got_entries(TCCState *s1)
                 relocs_info[type].gotplt_entry == AUTO_GOTPLT_ENTRY)
                 continue;
 
+            /* Building a dynamic library but target is not capable of PC
+               relative PLT entries. It can thus only use PLT entries if
+               it expects one to be used (ALWAYS_GOTPLT_ENTRY). */
+            if (sym->st_shndx == SHN_UNDEF &&
+                s1->output_type == TCC_OUTPUT_DLL &&
+                !PCRELATIVE_DLLPLT &&
+                relocs_info[type].gotplt_entry == AUTO_GOTPLT_ENTRY)
+                continue;
+
 #ifdef TCC_TARGET_X86_64
             if (type == R_X86_64_PLT32 &&
                 ELFW(ST_VISIBILITY)(sym->st_other) != STV_DEFAULT) {