riscv linux-user should align stack to 16-byte boundary

[michaeljclark]

+static abi_ulong get_sigframe(struct target_sigaction *ka,

•                          CPURISCVState *regs, size_t framesize)
  

+{

• abi_ulong sp = regs->gpr[xSP];
• int onsigstack = on_sig_stack(sp);
• /* redzone */
• /* This is the X/Open sanctioned signal stack switching. */
• if ((ka->sa_flags & TARGET_SA_ONSTACK) != 0 && !onsigstack) {

•    sp = target_sigaltstack_used.ss_sp + target_sigaltstack_used.ss_size;
  

• }
• sp -= framesize;
• sp &= ~3UL; /* align sp on 4-byte boundary */

kernel aligns using 0xf.

[michaeljclark]

Also:

1- check for signal stack overflow 2- check for X/Open sanctioned signal stack switching

http://lists.nongnu.org/archive/html/qemu-devel/2018-04/msg00338.html