[PATCH] x86/unwind/fp: Fix FP unwinding in ret_from_fork

[Josh Poimboeuf]

There have been some reports of "bad bp value" warnings printed by the
frame pointer unwinder:

  WARNING: kernel stack regs at 000000005bac7112 in sh:1014 has bad 'bp' value 0000000000000000

This warning happens when unwinding from an interrupt in
ret_from_fork().  If entry code gets interrupted, the state of the frame
pointer (rbp) may be undefined, which can confuse the unwinder,
resulting in warnings like the above.

There's an in_entry_code() check which normally silences such warnings
for entry code.  But in this case, ret_from_fork() is getting
interrupted.  It recently got moved out of .entry.text, so the
in_entry_code() check no longer works.

We could move it back into .entry.text, but that would break the noinstr
validation because of the call to schedule_tail().

Instead, initialize each new task's RBP to point to the task's entry
regs via an encoded frame pointer.  That will allow the unwinder to
reach the end of the stack gracefully.

Fixes: b9f6976bfb94 ("x86/entry/64: Move non entry code into .text section")
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Reported-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/include/asm/frame.h | 19 +++++++++++++++++++
 arch/x86/kernel/process.c    |  3 ++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/frame.h b/arch/x86/include/asm/frame.h
index 296b346184b2..fb42659f6e98 100644
--- a/arch/x86/include/asm/frame.h
+++ b/arch/x86/include/asm/frame.h
@@ -60,12 +60,26 @@
 #define FRAME_END "pop %" _ASM_BP "\n"
 
 #ifdef CONFIG_X86_64
+
 #define ENCODE_FRAME_POINTER			\
 	"lea 1(%rsp), %rbp\n\t"
+
+static inline unsigned long encode_frame_pointer(struct pt_regs *regs)
+{
+	return (unsigned long)regs + 1;
+}
+
 #else /* !CONFIG_X86_64 */
+
 #define ENCODE_FRAME_POINTER			\
 	"movl %esp, %ebp\n\t"			\
 	"andl $0x7fffffff, %ebp\n\t"
+
+static inline unsigned long encode_frame_pointer(struct pt_regs *regs)
+{
+	return (unsigned long)regs & 0x7fffffff;
+}
+
 #endif /* CONFIG_X86_64 */
 
 #endif /* __ASSEMBLY__ */
@@ -83,6 +97,11 @@
 
 #define ENCODE_FRAME_POINTER
 
+static inline unsigned long encode_frame_pointer(struct pt_regs *regs)
+{
+	return 0;
+}
+
 #endif
 
 #define FRAME_BEGIN
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 13ce616cc7af..ba4593a913fa 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -42,6 +42,7 @@
 #include <asm/spec-ctrl.h>
 #include <asm/io_bitmap.h>
 #include <asm/proto.h>
+#include <asm/frame.h>
 
 #include "process.h"
 
@@ -133,7 +134,7 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
 	fork_frame = container_of(childregs, struct fork_frame, regs);
 	frame = &fork_frame->frame;
 
-	frame->bp = 0;
+	frame->bp = encode_frame_pointer(childregs);
 	frame->ret_addr = (unsigned long) ret_from_fork;
 	p->thread.sp = (unsigned long) fork_frame;
 	p->thread.io_bitmap = NULL;
-- 
2.25.4