[PATCH v2 2/2] x86/kgdb: fix hang on failed breakpoint removal

[Florian Rommel <mail@florommel.de>]

On x86, occasionally, the removal of a breakpoint (i.e., removal of
the int3 instruction) fails because the text_mutex is taken by another
CPU (mainly due to the static_key mechanism, I think).  The function
kgdb_skipexception catches exceptions from these spurious int3
instructions, bails out of KGDB, and continues execution from the
previous PC address.

However, this led to an endless loop between the int3 instruction and
kgdb_skipexception since the int3 instruction (being still present)
triggered again.  This effectively caused the system to hang.

With this patch, we try to remove the concerned spurious int3
instruction in kgdb_skipexception before continuing execution.  This
may take a few attempts until the concurrent holders of the text_mutex
have released it, but eventually succeeds and the kernel can continue.

Signed-off-by: Florian Rommel <mail@florommel.de>
---
 arch/x86/kernel/kgdb.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 64c332151af7..585a7a72af74 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -723,7 +723,31 @@ void kgdb_arch_exit(void)
 int kgdb_skipexception(int exception, struct pt_regs *regs)
 {
 	if (exception == 3 && kgdb_isremovedbreak(regs->ip - 1)) {
+		struct kgdb_bkpt *bpt;
+		int i, error;
+
 		regs->ip -= 1;
+
+		/*
+		 * Try to remove the spurious int3 instruction.
+		 * These int3s can result from failed breakpoint removals
+		 * in kgdb_arch_remove_breakpoint.
+		 */
+		for (bpt = NULL, i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
+			if (kgdb_break[i].bpt_addr == regs->ip &&
+			    kgdb_break[i].state == BP_REMOVED &&
+			    (kgdb_break[i].type == BP_BREAKPOINT ||
+			     kgdb_break[i].type == BP_POKE_BREAKPOINT)) {
+				bpt = &kgdb_break[i];
+				break;
+			}
+		}
+		if (!bpt)
+			return 1;
+		error = kgdb_arch_remove_breakpoint(bpt);
+		if (error)
+			pr_err("skipexception: breakpoint remove failed: %lx\n",
+			       bpt->bpt_addr);
 		return 1;
 	}
 	return 0;
-- 
2.46.0