[PATCH] ia64: fix ptrace(PTRACE_GETREGS) (unbreaks strace, gdb)

[Sergei Trofimovich]

The strace breakage looks like that:
./strace: get_regs: get_regs_error: Input/output error

It happens because ia64 needs to load unwind tables
to read certain registers. Unwind tables fail to load
due to GCC quirk on the following code:

    extern char __end_unwind[];
    const struct unw_table_entry *end = (struct unw_table_entry *)table_end;
    table->end = segment_base + end[-1].end_offset;

GCC does not generate correct code for this single memory
reference after constant propagation (see https://gcc.gnu.org/PR84184).
Two triggers are required for bad code generation:
- '__end_unwind' has alignment lower (char), than
  'struct unw_table_entry' (8).
- symbol offset is negative.

This commit workarounds it by fixing alignment of '__end_unwind'.
While at it use hidden symbols to generate shorter gp-relative
relocations.

CC: Tony Luck <tony.luck@xxxxxxxxx>
CC: Fenghua Yu <fenghua.yu@xxxxxxxxx>
CC: linux-ia64@xxxxxxxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
Bug: https://github.com/strace/strace/issues/33
Bug: https://gcc.gnu.org/PR84184
Reported-by: Ãmeric Maschino <emeric.maschino@xxxxxxxxx>
Signed-off-by: Sergei Trofimovich <slyfox@xxxxxxxxxx>
---
 arch/ia64/include/asm/sections.h |  1 -
 arch/ia64/kernel/unwind.c        | 15 ++++++++++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/ia64/include/asm/sections.h b/arch/ia64/include/asm/sections.h
index f3481408594e..0fc4f1757a44 100644
--- a/arch/ia64/include/asm/sections.h
+++ b/arch/ia64/include/asm/sections.h
@@ -24,7 +24,6 @@ extern char __start_gate_mckinley_e9_patchlist[], __end_gate_mckinley_e9_patchli
 extern char __start_gate_vtop_patchlist[], __end_gate_vtop_patchlist[];
 extern char __start_gate_fsyscall_patchlist[], __end_gate_fsyscall_patchlist[];
 extern char __start_gate_brl_fsys_bubble_down_patchlist[], __end_gate_brl_fsys_bubble_down_patchlist[];
-extern char __start_unwind[], __end_unwind[];
 extern char __start_ivt_text[], __end_ivt_text[];
 
 #undef dereference_function_descriptor
diff --git a/arch/ia64/kernel/unwind.c b/arch/ia64/kernel/unwind.c
index e04efa088902..025ba6700790 100644
--- a/arch/ia64/kernel/unwind.c
+++ b/arch/ia64/kernel/unwind.c
@@ -2243,7 +2243,20 @@ __initcall(create_gate_table);
 void __init
 unw_init (void)
 {
-	extern char __gp[];
+	#define __ia64_hidden __attribute__((visibility("hidden")))
+	/*
+	 * We use hidden symbols to generate more efficient code using
+	 * gp-relative addressing.
+	 */
+	extern char __gp[] __ia64_hidden;
+	/*
+	 * Unwind tables need to have proper alignment as init_unwind_table()
+	 * uses negative offsets against '__end_unwind'.
+	 * See https://gcc.gnu.org/PR84184
+	 */
+	extern const struct unw_table_entry __start_unwind[] __ia64_hidden;
+	extern const struct unw_table_entry __end_unwind[] __ia64_hidden;
+	#undef __ia64_hidden
 	extern void unw_hash_index_t_is_too_narrow (void);
 	long i, off;
 
-- 
2.16.1