[PATCH v2] PowerPC: fix external calls from modules built by the compile command

[PATCH v2] PowerPC: fix external calls from modules built by the compile command

[Abhay Kandpal]

A module built for the compile command is loaded into inferior memory
without a linker, so it has no PLT.  On ELFv2 the default code model
still emits a direct bl for a call to another module, leaving r12 unset
and the caller's TOC pointer unrestored, and the callee then derives a
wrong r2 from a stale r12.

gdb.compile/compile-setjmp.exp shows this: the injected code calls
setjmp, whose global entry point computes r2 from a stale r12, and the
first TOC-relative load segfaults.

-mlongcall makes the call indirect, but on its own it reaches the target
through a PLT entry, giving R_PPC64_PLT16_HA/LO_DS relocations that BFD's
generic linker rejects.  Adding -mno-pltseq resolves the call against the
module's own TOC instead, via R_PPC64_TOC16_HA/LO_DS, which GDB already
handles.  GCC then emits the r2 save and restore around the call itself.

Tested on powerpc64le-linux; the two compile-setjmp.exp failures now pass
with no unexpected failures in the full testsuite.

Suggested-by: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
---
This patch is reg tested.

 gdb/ppc-linux-tdep.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/gdb/ppc-linux-tdep.c b/gdb/ppc-linux-tdep.c
index 8a55ff5d4af..b468dbcb62c 100644
--- a/gdb/ppc-linux-tdep.c
+++ b/gdb/ppc-linux-tdep.c
@@ -2196,6 +2196,18 @@ rs6000_linux_adjust_frame_regnum (struct gdbarch *gdbarch, int num,
   return num;
 }
 
+/* Implement gdbarch_gcc_target_options.  A module built for the "compile"
+   command is loaded into inferior memory without a linker, so it has no
+   PLT.  -mlongcall makes calls to other modules indirect, and -mno-pltseq
+   stops GCC reaching the target through a PLT entry; together they let the
+   call be resolved against the module's own TOC.  */
+
+static std::string
+ppc64_gcc_target_options (struct gdbarch *gdbarch)
+{
+  return default_gcc_target_options (gdbarch) + " -mlongcall -mno-pltseq";
+}
+
 static void
 ppc_linux_init_abi (struct gdbarch_info info,
 		    struct gdbarch *gdbarch)
@@ -2304,6 +2316,7 @@ ppc_linux_init_abi (struct gdbarch_info info,
 	    (gdbarch, ppc_elfv2_elf_make_msymbol_special);
 
 	  set_gdbarch_skip_entrypoint (gdbarch, ppc_elfv2_skip_entrypoint);
+	  set_gdbarch_gcc_target_options (gdbarch, ppc64_gcc_target_options);
 	}
 
       /* Shared library handling.  */
-- 
2.52.0

Re: [PATCH v2] PowerPC: fix external calls from modules built by the compile command

[Ulrich Weigand]

Abhay Kandpal <abhay@linux.ibm.com> wrote:

>A module built for the compile command is loaded into inferior memory
>without a linker, so it has no PLT.  On ELFv2 the default code model
>still emits a direct bl for a call to another module, leaving r12
unset
>and the caller's TOC pointer unrestored, and the callee then derives a
>wrong r2 from a stale r12.
>
>gdb.compile/compile-setjmp.exp shows this: the injected code calls
>setjmp, whose global entry point computes r2 from a stale r12, and the
>first TOC-relative load segfaults.
>
>-mlongcall makes the call indirect, but on its own it reaches the
target
>through a PLT entry, giving R_PPC64_PLT16_HA/LO_DS relocations that
BFD's
>generic linker rejects.  Adding -mno-pltseq resolves the call against
the
>module's own TOC instead, via R_PPC64_TOC16_HA/LO_DS, which GDB
already
>handles.  GCC then emits the r2 save and restore around the call
itself.

This is OK.

Thanks,
Ulrich