prog/alloc: align address allocation for aligned[addr]

[albertlinde]

AlignAttr was only used to set padding and not to align to the
given argument. Existing descriptions with [align[X]] don't have
an issue as they align to small blocks and default align is to 64.
This commits adds support for [align[X]] for an X larger than 64.

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https://github.com/google/syzkaller/blob/master/docs/contributing.md

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[codecov]

Codecov Report

Merging #1894 into master will increase coverage by 0.0%.
The diff coverage is 86.8%.

Impacted Files	Coverage Δ
prog/target.go	62.3% <0.0%> (+0.8%)	⬆️
prog/alloc.go	77.0% <50.0%> (-1.6%)	⬇️
prog/types.go	71.9% <66.7%> (-0.1%)	⬇️
pkg/compiler/gen.go	93.9% <100.0%> (+0.4%)	⬆️
pkg/compiler/types.go	96.4% <100.0%> (+0.3%)	⬆️
prog/encoding.go	84.9% <100.0%> (ø)
prog/rand.go	92.3% <100.0%> (ø)
tools/syz-trace2syz/proggen/proggen.go	62.2% <100.0%> (ø)

[melver]

Thank you!

[albertlinde]

@dvyukov I've tried to remove AlignAttr from StructType to use TypeAlign instead, but there are places where it is used -- and where using TypeAlign gives different semantics.
TypeAlign must depend on the alignment of inner elements, but for example here: any.go and prog.go, padding depends on the AlignAttr attribute.

It seems to me that we need two variables. If we use one and it is set to for example 4, we can't know if it is due inner elements or the attribute.

[dvyukov]

@dvyukov I've tried to remove AlignAttr from StructType to use TypeAlign instead, but there are places where it is used -- and where using TypeAlign gives different semantics.

OK, let's keep.
But at least we don't need to add PackedAttr, right? We did not use it before.

TypeAlign must depend on the alignment of inner elements,

If there is alignment attribute, then it overrides any filed alignment:
https://github.com/google/syzkaller/blob/master/pkg/compiler/gen.go#L454

but I think there still may be some differences for variable-sized structs:
with explicit align attribute we will add padding after the struct dynamically (your prog.go link)

but for example here: any.go and prog.go, padding depends on the AlignAttr attribute.

It seems to me that we need two variables. If we use one and it is set to for example 4, we can't know if it is due inner elements or the attribute.

[dvyukov]

Nice! That's a useful capability that nicely fits into the code base. Thanks.