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Subject[PATCH] ia64: fix ptrace(PTRACE_GETREGS) (unbreaks strace, gdb)
Date
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@intel.com>
CC: Fenghua Yu <fenghua.yu@intel.com>
CC: linux-ia64@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Bug: https://github.com/strace/strace/issues/33
Bug: https://gcc.gnu.org/PR84184
Reported-by: Émeric Maschino <emeric.maschino@gmail.com>
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
---
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
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