On Tue, Dec 14, 2021 at 12:40 PM Manuel Lauss via Gcc-help <gcc-help@xxxxxxxxxxx> wrote: > Is there an explanation of how this whole stack magic works? I wrote > a small test for dlvsym() and its stack pointer is precisely > aligned so that the vmovdqa with displacement 0xc points to a 16-byte > aligned address. However I don't get how the failing program gets its > misaligned stack. Does it have to be built with special compiler flags or > linker scripts? No magic involved. The rule is that you are only allowed to invoke C functions if $esp is divisible by 16 before execution of the call instruction. $esp is forced to an aligned address at the beginning of main, and the compiler will not generate functions that create odd stack frames, so it will stay aligned at all times without further runtime penalties. The only price to pay is a minuscule increase of 8 bytes in stack frame size for some functions compared to the previous rules (I think 4-byte alignment was never allowed).
