linux-kernel - [PATCH AUTOSEL 6.1 30/53] tools/nolibc: prevent gcc from making memset() loop over itself

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Message-Id: <20230116140154.114951-30-sashal@kernel.org>
Date:   Mon, 16 Jan 2023 09:01:30 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Willy Tarreau <w@....eu>, "Paul E . McKenney" <paulmck@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 6.1 30/53] tools/nolibc: prevent gcc from making memset() loop over itself

From: Willy Tarreau <w@....eu>

[ Upstream commit 1bfbe1f3e96720daf185f03d101f072d69753f88 ]

When building on ARM in thumb mode with gcc-11.3 at -O2 or -O3,
nolibc-test segfaults during the select() tests. It turns out that at
this level, gcc recognizes an opportunity for using memset() to zero
the fd_set, but it miscompiles it because it also recognizes a memset
pattern as well, and decides to call memset() from the memset() code:

  000122bc <memset>:
     122bc:       b510            push    {r4, lr}
     122be:       0004            movs    r4, r0
     122c0:       2a00            cmp     r2, #0
     122c2:       d003            beq.n   122cc <memset+0x10>
     122c4:       23ff            movs    r3, #255        ; 0xff
     122c6:       4019            ands    r1, r3
     122c8:       f7ff fff8       bl      122bc <memset>
     122cc:       0020            movs    r0, r4
     122ce:       bd10            pop     {r4, pc}

Simply placing an empty asm() statement inside the loop suffices to
avoid this.

Signed-off-by: Willy Tarreau <w@....eu>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/include/nolibc/string.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h
index 0932db3817d2..fffdaf6ff467 100644
--- a/tools/include/nolibc/string.h
+++ b/tools/include/nolibc/string.h
@@ -88,8 +88,11 @@ void *memset(void *dst, int b, size_t len)
 {
 	char *p = dst;
 
-	while (len--)
+	while (len--) {
+		/* prevent gcc from recognizing memset() here */
+		asm volatile("");
 		*(p++) = b;
+	}
 	return dst;
 }
 
-- 
2.35.1