Hmm, this is a surprising bug. syzbot provided a C reproducer, but the definition
of "struct serial_struct" used in that reproducer is wrong. As a result, syzbot was
reporting crash caused by passing wrong arguments. ;-)

close_delay field used in the C reproducer is sizeof(unsigned int) bytes rather than
sizeof(unsigned short) bytes, thus fields after close_delay field are incorrectly
interpreted.

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#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/serial.h>

struct bad_serial_struct {
        int     type;
        int     line;
        unsigned int    port;
        int     irq;
        int     flags;
        int     xmit_fifo_size;
        int     custom_divisor;
        int     baud_base;
        unsigned int    close_delay; /* Correct type is "unsigned short". */
        char    io_type;
        char    reserved_char[1];
        int     hub6;
        unsigned short  closing_wait;
        unsigned short  closing_wait2;
        unsigned char   *iomem_base;
        unsigned short  iomem_reg_shift;
        unsigned int    port_high;
        unsigned long   iomap_base;
};

int main(int argc, char *argv[])
{
        struct bad_serial_struct ss = { };
        int fd = open("/dev/ttyS3", O_RDONLY);
        ss.type = 0xa;
        ss.line = 0x400000;
        ss.port = 0x100;
        ss.irq = 0;
        ss.flags = 0x400000;
        ss.xmit_fifo_size = 0;
        ss.custom_divisor = 0;
        ss.baud_base = 0x80000;
        ss.close_delay = 0x200ff;
        ss.io_type = 0;
        ss.reserved_char[0] = 0x41;
        ss.hub6 = 3;
        ss.closing_wait = 0;
        ss.closing_wait2 = 0x7c5;
        ss.iomem_base = NULL;
        ss.iomem_reg_shift = 0;
        ss.port_high = 0;
        ss.iomap_base = 0;
        ioctl(fd, TIOCSSERIAL, &ss);
        return 0;
}
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