From 4f242fc5f2e24412b89e934dad025b10293b2712 Mon Sep 17 00:00:00 2001
From: Nicholas Piggin <npiggin@gmail.com>
Date: Sat, 8 May 2021 20:14:55 +1000
Subject: [PATCH] powerpc/pseries: warn if recursing into the hcall tracing
 code

References: bsc#1185110 ltc#192091
Patch-mainline: v5.13-rc2
Git-commit: 4f242fc5f2e24412b89e934dad025b10293b2712

The hcall tracing code has a recursion check built in, which skips
tracing if we are already tracing an hcall.

However if the tracing code has problems with recursion, this check
may not catch all cases because the tracing code could be invoked from
a different tracepoint first, then make an hcall that gets traced,
then recurse.

Add an explicit warning if recursion is detected here, which might help
to notice tracing code making hcalls. Really the core trace code should
have its own recursion checking and warnings though.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210508101455.1578318-5-npiggin@gmail.com
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
 arch/powerpc/platforms/pseries/lpar.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index ad1cec80019b..dab356e3ff87 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -1829,8 +1829,14 @@ void hcall_tracepoint_unregfunc(void)
 #endif
 
 /*
- * Since the tracing code might execute hcalls we need to guard against
- * recursion.
+ * Keep track of hcall tracing depth and prevent recursion. Warn if any is
+ * detected because it may indicate a problem. This will not catch all
+ * problems with tracing code making hcalls, because the tracing might have
+ * been invoked from a non-hcall, so the first hcall could recurse into it
+ * without warning here, but this better than nothing.
+ *
+ * Hcalls with specific problems being traced should use the _notrace
+ * plpar_hcall variants.
  */
 static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
 
@@ -1844,7 +1850,7 @@ notrace void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
 
 	depth = this_cpu_ptr(&hcall_trace_depth);
 
-	if (*depth)
+	if (WARN_ON_ONCE(*depth))
 		goto out;
 
 	(*depth)++;
@@ -1865,7 +1871,7 @@ notrace void __trace_hcall_exit(long opcode, long retval, unsigned long *retbuf)
 
 	depth = this_cpu_ptr(&hcall_trace_depth);
 
-	if (*depth)
+	if (*depth) /* Don't warn again on the way out */
 		goto out;
 
 	(*depth)++;
-- 
2.26.2