RE: [PATCH] crypto: ctr: avoid VLA use

From: Eric Biggers

Sent: 14 March 2018 18:32

...

Also, I recall there being a long discussion a while back about how
__aligned(16) doesn't work on local variables because the kernel's stack pointer
isn't guaranteed to maintain the alignment assumed by the compiler (see commit
b8fbe71f7535)...

ISTR that gcc arbitrarily decided that the x86 stack (for 32 bit) would be
kept aligned to more than 4 bytes (16??) - probably so that xmm registers
could be written to stack locations.
This was a massive ABI change that they didn't tell anyone about!
While gcc compiled code maintained the alignment a lot of asm code didn't.
I don't know about Linux, but NetBSD didn't even align user stacks.

There is a gcc option to not assume that the stack is 'appropriately aligned',
but ISTR that it generates rather more code that one might have wished.

If the compiler does align the stack, it does so by generating a double
stack frame - not pretty at all.

	David