Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 25 Oct 2022 11:38:31 -0700 | Subject | Re: [PATCH 1/2] bitmap: add sanity check function for find_bit() |
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On Tue, Oct 25, 2022 at 11:26 AM Russell King (Oracle) <linux@armlinux.org.uk> wrote: > > However, we still support ARMv5 and ARMv4, both of which _trap_ every > unaligned access, which will make a findbit call with an unaligned > pointer using word loads painfully expensive. This is the main reason > we haven't used word loads in the findbit ops.
The findbit ops really shouldn't be a special case, and bitmaps can never be unaligned.
Just look at what 'test_bit()' does: the non-constant non-instrumented version ends up as generic_test_bit(), which uses a "const volatile unsigned long *" access to do the bitmap load.
So there is absolutely no way that bitmaps can ever be unaligned, because that would trap.
And test_bit() is a lot more fundamental than one of the "find bits" functions.
Have we had bugs in this area before? Sure. People have used "unsigned int" for flags and mised the bitmap ops on it, and it has worked on x86.
But then it fails *miserably* on big-endian machines and on machines that require more alignment (and even on x86 we have KASAN failures etc these days and obviously without casts it will warn), so we've hopefully fixed all those cases up long long ago.
So I really think it's pointless to worry about alignment for "find_bit()" and friends, when much more fundamental bitop functions don't worry about it.
Linus
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