Copying data in the kernel

Question

I want to copy a string into a local buffer using copystr() but have been having problems doing so. The string I want to copy is uap->path from the VFS function sys_chdir in /usr/src/sys/kern/vfs_syscalls.c. Here is one of the things I've tried:

#define USR_MAX_LEN 22
char mypath[USR_MAX_LEN + 1];
size_t len = USR_MAX_LEN;
size_t done;

copystr(uap->path, mypath, len, &done);

Unfortunately when I do this I end up getting a kernel panic and a subsequent crash. It always crashes in copystr():

(kgdb) list *0xffffffff80c8c860
0xffffffff80c8c860 is at /usr/src/sys/amd64/amd64/support.S:606.
601             cld
602     1:
603             decq    %rdx
604             jz      4f
605             lodsb
606             stosb
607             orb     %al,%al
608             jnz     1b
609
610             /* Success -- 0 byte reached */
Current language:  auto; currently minimal
(kgdb) bt
#0  doadump (textdump=<value optimized out>) at pcpu.h:219
#1  0xffffffff808af530 in kern_reboot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:447
#2  0xffffffff808af8f4 in panic (fmt=<value optimized out>) at /usr/src/sys/kern/kern_shutdown.c:754
#3  0xffffffff80c8e6d2 in trap_fatal (frame=<value optimized out>, eva=<value optimized out>)
at /usr/src/sys/amd64/amd64/trap.c:882
#4  0xffffffff80c8e9a9 in trap_pfault (frame=0xfffffe004e8ed9f0, usermode=0) at /usr/src/sys/amd64/amd64/trap.c:699
#5  0xffffffff80c8e136 in trap (frame=0xfffffe004e8ed9f0) at /usr/src/sys/amd64/amd64/trap.c:463
#6  0xffffffff80c753d2 in calltrap () at /usr/src/sys/amd64/amd64/exception.S:232
#7  0xffffffff80c8c860 in copystr () at /usr/src/sys/amd64/amd64/support.S:605
#8  0xffffffff80950e64 in sys_chdir (td=0xfffff80002782000, uap=0xfffffe004e8edb80) at /usr/src/sys/kern/vfs_syscalls.c:838
#9  0xffffffff80c8efc7 in amd64_syscall (td=0xfffff80002782000, traced=0) at subr_syscall.c:134
#10 0xffffffff80c756bb in Xfast_syscall () at /usr/src/sys/amd64/amd64/exception.S:391
#11 0x0000000800d1fc0a in ?? ()
Previous frame inner to this frame (corrupt stack?)
(kgdb) 

What am I doing wrong here? As far as I can tell my data types satisfy the ones from the man page for copystr so I don't think it's that. Any help is appreciated. Thanks.

Answer 1

The man page for copystr says:

The copystr() function copies a NUL-terminated string, at most len bytes long, from kernel-space address kfaddr to kernel-space address kdaddr.

But it looks like you are trying to copy from userspace. In that case I think you are looking for copyin or copyinstr.