Re: [PATCH] media: mali-c55: Fix unaligned access of AEC histogram zone weights

[David CARLIER]

> Out of curiosity: why is (u32 *) case a UB ?

Alignment, not aliasing. zone_weights is a u8[] at offset 10, so it's
only 2-byte aligned, and casting that to a u32* (which wants 4) is
already UB - 6.3.2.3p7 - before you even load through it.

> the usage of __packed triggers the compiler to emit an 'LDUR'
> ... implications of using LDUR vs LDR on "unaligned access" ... not
> 100% clear to me.

LDUR vs LDR is only about how the offset is encoded, it's got nothing to
do with alignment safety. LDR's scaled form needs the immediate to be a
multiple of the access size, +10 isn't, so gcc can't use it and drops to
LDUR (unscaled offset). Both happily load from an unaligned address on
arm64 with SCTLR.A off - LDUR isn't "the unaligned one". The multiple-of-4
you found is about the immediate field, not the address.

So on arm64 __packed doesn't buy you a safer load, the plain cast already
worked. What it buys you is not lying to the compiler about the alignment
(so the UB is gone), plus correct codegen on the arches that do trap -
which is David's point.

> I would be a bit hesitant in changing the uAPI if there is actually
> nothing broken ... happy to defer

Fair enough, and you're right that nothing's actually broken - it's arm64
only so it never faults, this is tidy-up not a fix. I don't feel strongly
either way. Leave the uAPI as is and I'll drop it, or if you'd rather have
it cleaned up I'll send the packed union (with MALI_C55_MAX_ZONES / 4 like
David said). Whatever you prefer.

Cheers