linux-kernel - Re: [PATCH] nbd: use shifts rather than multiplies

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Date:   Wed, 29 Sep 2021 20:32:07 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Josef Bacik <josef@...icpanda.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Arnd Bergmann <arnd@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Machek <pavel@....cz>, linux-block@...r.kernel.org,
        nbd@...er.debian.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] nbd: use shifts rather than multiplies

On 9/20/21 5:25 PM, Nick Desaulniers wrote:
> commit fad7cd3310db ("nbd: add the check to prevent overflow in
> __nbd_ioctl()") raised an issue from the fallback helpers added in
> commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and
> add fallback code")
> 
> ERROR: modpost: "__divdi3" [drivers/block/nbd.ko] undefined!
> 
> As Stephen Rothwell notes:
>   The added check_mul_overflow() call is being passed 64 bit values.
>   COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW is not set for this build (see
>   include/linux/overflow.h).
> 
> Specifically, the helpers for checking whether the results of a
> multiplication overflowed (__unsigned_mul_overflow,
> __signed_add_overflow) use the division operator when
> !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW.  This is problematic for 64b
> operands on 32b hosts.
> 
> This was fixed upstream by
> commit 76ae847497bc ("Documentation: raise minimum supported version of
> GCC to 5.1")
> which is not suitable to be backported to stable.
> 
> Further, __builtin_mul_overflow() would emit a libcall to a
> compiler-rt-only symbol when compiling with clang < 14 for 32b targets.
> 
> ld.lld: error: undefined symbol: __mulodi4
> 
> In order to keep stable buildable with GCC 4.9 and clang < 14, modify
> struct nbd_config to instead track the number of bits of the block size;
> reconstructing the block size using runtime checked shifts that are not
> problematic for those compilers and in a ways that can be backported to
> stable.
> 
> In nbd_set_size, we do validate that the value of blksize must be a
> power of two (POT) and is in the range of [512, PAGE_SIZE] (both
> inclusive).
> 
> This does modify the debugfs interface.

Applied, thanks.

-- 
Jens Axboe