linux-kernel - [PATCH] asm-generic: fix __get_unaligned

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Message-ID: <20220406233909.529613-1-alobakin@pm.me>
Date:   Wed, 06 Apr 2022 23:46:04 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Bart Van Assche <bvanassche@....org>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <kbusch@...nel.org>,
        Chaitanya Kulkarni <kch@...dia.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexander Lobakin <alobakin@...me>
Subject: [PATCH] asm-generic: fix __get_unaligned_be48() on 32 bit platforms

While testing the new macros for working with 48 bit containers,
I faced a weird problem:

32 + 16: 0x2ef6e8da 0x79e60000
48: 0xffffe8da + 0x79e60000

All the bits starting from the 32nd were getting 1d in 9/10 cases.
The debug showed:

p[0]: 0x00002e0000000000
p[1]: 0x00002ef600000000
p[2]: 0xffffffffe8000000
p[3]: 0xffffffffe8da0000
p[4]: 0xffffffffe8da7900
p[5]: 0xffffffffe8da79e6

that the value becomes a garbage after the third OR, i.e. on
`p[2] << 24`.
When the 31st bit is 1 and there's no explicit cast to an unsigned,
it's being considered as a signed int and getting sign-extended on
OR, so `e8000000` becomes `ffffffffe8000000` and messes up the
result.
Cast the @p[2] to u64 as well to avoid this. Now:

32 + 16: 0x7ef6a490 0xddc10000
48: 0x7ef6a490 + 0xddc10000

p[0]: 0x00007e0000000000
p[1]: 0x00007ef600000000
p[2]: 0x00007ef6a4000000
p[3]: 0x00007ef6a4900000
p[4]: 0x00007ef6a490dd00
p[5]: 0x00007ef6a490ddc1

Fixes: c2ea5fcf53d5 ("asm-generic: introduce be48 unaligned accessors")
Signed-off-by: Alexander Lobakin <alobakin@...me>
---
 include/asm-generic/unaligned.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unaligned.h
index 8fc637379899..df30f11b4a46 100644
--- a/include/asm-generic/unaligned.h
+++ b/include/asm-generic/unaligned.h
@@ -143,7 +143,7 @@ static inline void put_unaligned_be48(const u64 val, void *p)

 static inline u64 __get_unaligned_be48(const u8 *p)
 {
-	return (u64)p[0] << 40 | (u64)p[1] << 32 | p[2] << 24 |
+	return (u64)p[0] << 40 | (u64)p[1] << 32 | (u64)p[2] << 24 |
 		p[3] << 16 | p[4] << 8 | p[5];
 }

--
2.35.1