+++ Johan Hovold [03/11/20 18:57 +0100]:
We rely on the linker to create arrays for a number of things including kernel parameters and device-tree-match entries. The stride of these linker-section arrays obviously needs to match the expectations of the code accessing them or bad things will happen. One thing to watch out for is that gcc is known to increase the alignment of larger objects with static extent as an optimisation (on x86), but this can be suppressed by using the aligned attribute when declaring entries. We've been relying on this behaviour for 16 years for kernel parameters (and other structures) and it indeed hasn't changed since the introduction of the aligned attribute in gcc 3.1 (see align_variable() in [1]). Occasionally this gcc optimisation do cause problems which have instead been worked around in various creative ways including using indirection through an array of pointers. This was originally done for tracepoints [2] after a number of failed attempts to create properly aligned arrays, and the approach was later reused for module-version attributes [3] and earlycon entries.
[2] https://lore.kernel.org/lkml/20110126222622.GA10794@Krystal/
Hi Johan, So unfortunately, I am not familiar enough with the semantics of gcc's aligned attribute. AFAICT from the patch you linked in [2], the original purpose of the pointer indirection workaround was to avoid relying on (potentially inconsistent) compiler-specific behavior with respect to the aligned attribute. The main concern was potential up-alignment being done by gcc (or the linker) despite the desired alignment being specified. Indeed, the gcc documentation also states that the aligned attribute only specifies the *minimum* alignment, although there's no guarantee that up-alignment wouldn't occur. So I guess my question is, is there some implicit guarantee that specifying alignment by type via __alignof__ that's supposed to prevent gcc from up-aligning? Or are we just assuming that gcc won't increase the alignment? The gcc docs don't seem to clarify this unfortunately. Thanks, Jessica
