rtld: populate DT_DEBUG iff DYNAMIC segment is writable

MIPS has/had a read-only DYNAMIC segment, and uses an extra level of
indirection (through MIPS_RLD_MAP) to locate the debugger rendezvous
data.

Some linkers (e.g. LLVM's lld) may produce MIPS binaries with a writable
DYNAMIC segment, which would allow us to eventually drop a special case.

Therefore, instead of hardcoding knowledge that DYNAMIC is not writable
on MIPS just check the permissions on the segment.

Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D4791

1 files changed, 4 insertions, 2 deletions

diff --git a/libexec/rtld-elf/rtld.c b/libexec/rtld-elf/rtld.c
index 66edc157d393..6daef2d13405 100644
--- a/libexec/rtld-elf/rtld.c
+++ b/libexec/rtld-elf/rtld.c
@@ -1144,13 +1144,13 @@ digest_dynamic1(Obj_Entry *obj, int early, const Elf_Dyn **dyn_rpath,
 	 * is mapped read-only. DT_MIPS_RLD_MAP is used instead.
 	 */
 
-#ifndef __mips__
 	case DT_DEBUG:
+	    if (!obj->writable_dynamic)
+		break;
 	    if (!early)
 		dbg("Filling in DT_DEBUG entry");
 	    ((Elf_Dyn*)dynp)->d_un.d_ptr = (Elf_Addr) &r_debug;
 	    break;
-#endif
 
 	case DT_FLAGS:
 		if (dynp->d_un.d_val & DF_ORIGIN)
@@ -1331,6 +1331,8 @@ digest_phdr(const Elf_Phdr *phdr, int phnum, caddr_t entry, const char *path)
 	    break;
 
 	case PT_DYNAMIC:
+	    if (ph->p_flags & PROT_WRITE)
+		obj->writable_dynamic = true;
 	    obj->dynamic = (const Elf_Dyn *)(ph->p_vaddr + obj->relocbase);
 	    break;