The problem is something a bit else (jstarks figured it out somewhere below). I'm not a compiler/abi eng, but it seems to depend on a compiler, eg. consider this with clang-16:

  #include <sys/syscall.h>
  #include <unistd.h>
  #include <alloca.h>
  #include <string.h>
  
  void s(long a, long b, long c, long d, long e, long f, long g) {
  }
  
  int main(void) {
   long a = 0xFFFFFFFFFFFFFFFF;
   s(a, a, a, a, a, a, a);
   syscall(9999, 1, 2, 3, 4, 5, 6);
   return 0;
  }

Now, strace shows:

  $ strace -e process_vm_readv ./a
  process_vm_readv(1, 0x2, 3, 0x4, 5, 18446744069414584326) = -1 EINVAL (Invalid argument)

objdump -d a

  117f: 48 c7 45 f0 ff ff ff  movq   $0xffffffffffffffff,-0x10(%rbp)
  1186: ff 
  1187: 48 8b 7d f0           mov    -0x10(%rbp),%rdi
  118b: 48 8b 75 f0           mov    -0x10(%rbp),%rsi
  118f: 48 8b 55 f0           mov    -0x10(%rbp),%rdx
  1193: 48 8b 4d f0           mov    -0x10(%rbp),%rcx
  1197: 4c 8b 45 f0           mov    -0x10(%rbp),%r8
  119b: 4c 8b 4d f0           mov    -0x10(%rbp),%r9
  119f: 48 8b 45 f0           mov    -0x10(%rbp),%rax
  11a3: 48 89 04 24           mov    %rax,(%rsp)
  11a7: e8 94 ff ff ff        call   1140 <s>
  11ac: bf 36 01 00 00        mov    $0x136,%edi
  11b1: be 01 00 00 00        mov    $0x1,%esi
  11b6: ba 02 00 00 00        mov    $0x2,%edx
  11bb: b9 03 00 00 00        mov    $0x3,%ecx
  11c0: 41 b8 04 00 00 00     mov    $0x4,%r8d
  11c6: 41 b9 05 00 00 00     mov    $0x5,%r9d
  11cc: c7 04 24 06 00 00 00  movl   $0x6,(%rsp)
  11d3: b0 00                 mov    $0x0,%al
  11d5: e8 56 fe ff ff        call   1030 <syscall@plt>

Only 4 bytes are put on the stack, but syscall will read 8.

It's tricky if one doesn't control types of arguments used in vararg.