linux-user: --strace incorrectly decodes writev arguments for 64-bit binaries on 32-bit machine

Host environment

• Operating system: Gentoo
• OS/kernel version: Linux t60 5.10.88-gentoo-dist #1 SMP Wed Dec 22 15:58:38 -00 2021 i686 Genuine Intel(R) CPU T2400 @ 1.83GHz GenuineIntel GNU/Linux
• Architecture: i686
• QEMU flavor: qemu-aarch64, qemu-x86_64
• QEMU version: 6.2.0
• QEMU command line: qemu-x86_64 --strace ./aarch64-main

Emulated/Virtualized environment

• Operating system: linux-user
• OS/kernel version: linux-user
• Architecture: x86_64, aarch64

Description of problem

With --strace, the arguments to writev appear to be decoded incorrectly. The syscall still succeeds and has the expected effects.

Steps to reproduce

$ cat main.c
#include <sys/uio.h>

int main(void) {
  struct iovec iov;
  iov.iov_base = "hello, world!\n";
  iov.iov_len = 14;
  return writev(1, &iov, 1);
}

$ aarch64-unknown-linux-gnu-gcc -static -o aarch64-main main.c

$ x86_64-pc-linux-gnu-gcc -static -o x86_64-main main.c

$ i686-pc-linux-gnu-gcc -static -o i686-main main.c

$ ./i686-main
hello, world!

$ strace ./i686-main |& grep writev
writev(1, [{iov_base="hello, world!\n", iov_len=14}], 1hello, world!

$ qemu-i386 --strace ./i686-main |& grep writev
21953 writev(1,0x407ffe54,0x1) = 14

$ qemu-x86_64 --strace ./x86_64-main |& grep writev
22218 writev(1,(nil),0x407ffcc0) = 14

$ qemu-aarch64 --strace ./aarch64-main |& grep writev
22523 writev(1,(nil),0x407ffcc8) = 14