linux-kernel - [PATCH 1/2] ftrace: Drop duplicate mcount locations

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Message-Id: <9b1b816cff1f479c8de0e9baa5a6ac680b84e17e.1651047542.git.naveen.n.rao@linux.vnet.ibm.com>
Date:   Wed, 27 Apr 2022 15:01:21 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        llvm@...ts.linux.dev
Subject: [PATCH 1/2] ftrace: Drop duplicate mcount locations

In the absence of section symbols [1], objtool (today) and recordmcount
(with a subsequent patch) generate __mcount_loc relocation records with
weak symbols as the base. This works fine as long as those weak symbols
are not overridden, but if they are, these can result in duplicate
entries in the final vmlinux mcount location table. This will cause
ftrace to fail when trying to patch the same location twice. Fix this by
dropping duplicate locations during ftrace init.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1

Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
---
 kernel/trace/ftrace.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 4f1d2f5e726341..8bc4f282bb3ff4 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6496,7 +6496,7 @@ static int ftrace_process_locs(struct module *mod,
 	struct dyn_ftrace *rec;
 	unsigned long count;
 	unsigned long *p;
-	unsigned long addr;
+	unsigned long addr, prev_addr = 0;
 	unsigned long flags = 0; /* Shut up gcc */
 	int ret = -ENOMEM;
 
@@ -6550,6 +6550,16 @@ static int ftrace_process_locs(struct module *mod,
 	while (p < end) {
 		unsigned long end_offset;
 		addr = ftrace_call_adjust(*p++);
+
+		/*
+		 * Drop duplicate entries, which can happen when weak
+		 * functions are overridden, and __mcount_loc relocation
+		 * records were generated against function names due to
+		 * absence of non-weak section symbols
+		 */
+		if (addr == prev_addr)
+			addr = 0;
+
 		/*
 		 * Some architecture linkers will pad between
 		 * the different mcount_loc sections of different
@@ -6569,6 +6579,7 @@ static int ftrace_process_locs(struct module *mod,
 
 		rec = &pg->records[pg->index++];
 		rec->ip = addr;
+		prev_addr = addr;
 	}
 
 	/* We should have used all pages */
-- 
2.35.1