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From:   Andreas Schwab <schwab@linux-m68k.org>
To:     Emil Renner Berthing <emil.renner.berthing@canonical.com>
Cc:     Charlie Jenkins <charlie@rivosinc.com>,
        linux-riscv@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Samuel Holland <samuel.holland@sifive.com>,
        Nelson Chu <nelson@rivosinc.com>,
        Emil Renner Berthing <kernel@esmil.dk>
Subject: Re: [PATCH v7 1/3] riscv: Avoid unaligned access when relocating
 modules
In-Reply-To: <CAJM55Z-v0EwrZp876DdLSzad2u55nJV_uBs_+MUJDqFW5MTPkA@mail.gmail.com>
        (Emil Renner Berthing's message of "Tue, 31 Oct 2023 06:11:47 -0700")
References: <20231031-module_relocations-v7-0-6f4719b64bf7@rivosinc.com>
        <20231031-module_relocations-v7-1-6f4719b64bf7@rivosinc.com>
        <CAJM55Z-v0EwrZp876DdLSzad2u55nJV_uBs_+MUJDqFW5MTPkA@mail.gmail.com>
Date:   Tue, 31 Oct 2023 17:35:57 +0100
Message-ID: <878r7iiwb6.fsf@igel.home>
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On Okt 31 2023, Emil Renner Berthing wrote:

>> +static int riscv_insn_rmw(void *location, u32 keep, u32 set)
>> +{
>> +	u16 *parcel = location;
>> +	u32 insn = (u32)le16_to_cpu(parcel[0]) | (u32)le16_to_cpu(parcel[1]) << 16;
>> +
>> +	insn &= keep;
>> +	insn |= set;
>> +
>> +	parcel[0] = cpu_to_le32(insn);
>
> Why cpu_to_le32(insn)? Unless I've misunderstood something downcasting unsigned
> to unsigned values in C (eg. from u32 to u16) is defined to always discard the
> most signifcant bits, so cpu_to_le16(insn) should be fine.

cpu_to_le32(insn) can't be right here anyway, since it also swaps the
two u16 halves and would be the same as cpu_to_le16(insn >> 16) on big
endian.

-- 
Andreas Schwab, schwab@linux-m68k.org
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"And now for something completely different."