Messages in this thread Patch in this message | | | From | Sergei Trofimovich <> | Subject | [PATCH v2, simpler] ia64: fix ptrace(PTRACE_GETREGS) (unbreaks strace, gdb) | Date | Fri, 9 Mar 2018 23:15:55 +0000 |
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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 in 'PTRACE_GETREGS'. Unwind tables fail to load at kernel startup due to GCC quirk on the following code (logged as PR 84184):
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. 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 disabling inline on init_unwind_table(). This way we avoid const-propagation of '__end_unwind' and pass address via register.
Tested in ski (emulator) and on rx2600, rx3600 (real hardware). In case of rx2600 it unbreaks booting.
This patch is a lighter version of patch https://lkml.org/lkml/2018/2/2/914
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> Tested-by: stanton_arch@mail.com Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org> --- arch/ia64/kernel/unwind.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/arch/ia64/kernel/unwind.c b/arch/ia64/kernel/unwind.c index e04efa088902..a18190bc99a9 100644 --- a/arch/ia64/kernel/unwind.c +++ b/arch/ia64/kernel/unwind.c @@ -2078,7 +2078,14 @@ unw_init_from_blocked_task (struct unw_frame_info *info, struct task_struct *t) } EXPORT_SYMBOL(unw_init_from_blocked_task); -static void +/* + * We use 'noinline' to evade GCC bug https://gcc.gnu.org/PR84184 + * where gcc code generator emits incorrect code when '__end_unwind' + * is const-propagated to 'end[-1].end_offset' and gcc generates + * incorrect code. The prigger there is negative offset relative + * to externally-defined symbol. + */ +noinline static void init_unwind_table (struct unw_table *table, const char *name, unsigned long segment_base, unsigned long gp, const void *table_start, const void *table_end) { -- 2.16.2
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