x86/unwind/orc: Fix ORC for newly forked tasks

The ORC unwinder fails to unwind newly forked tasks which haven't yet
run on the CPU.  It correctly reads the 'ret_from_fork' instruction
pointer from the stack, but it incorrectly interprets that value as a
call stack address rather than a "signal" one, so the address gets
incorrectly decremented in the call to orc_find(), resulting in bad ORC
data.

Fix it by forcing 'ret_from_fork' frames to be signal frames.

Reported-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Link: https://lkml.kernel.org/r/f91a8778dde8aae7f71884b5df2b16d552040441.1594994374.git.jpoimboe@redhat.com

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index 7f969b2..ec88bbe 100644 (file)
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -440,8 +440,11 @@ bool unwind_next_frame(struct unwind_state *state)
        /*
         * Find the orc_entry associated with the text address.
         *
-        * Decrement call return addresses by one so they work for sibling
-        * calls and calls to noreturn functions.
+        * For a call frame (as opposed to a signal frame), state->ip points to
+        * the instruction after the call.  That instruction's stack layout
+        * could be different from the call instruction's layout, for example
+        * if the call was to a noreturn function.  So get the ORC data for the
+        * call instruction itself.
         */
        orc = orc_find(state->signal ? state->ip : state->ip - 1);
        if (!orc) {
@@ -662,6 +665,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
                state->sp = task->thread.sp;
                state->bp = READ_ONCE_NOCHECK(frame->bp);
                state->ip = READ_ONCE_NOCHECK(frame->ret_addr);
+               state->signal = (void *)state->ip == ret_from_fork;
        }
 
        if (get_stack_info((unsigned long *)state->sp, state->task,