Error while building a kernel module with mkdir(2)

[Rocky Hotas]

Hello!
I hope this is the right ML to discuss about this issue. I am trying to
build the example kernel module presented in paragraph 2.1 of

Designing BSD Rootkits: An Introduction to Kernel Hacking
by Joseph Kong

However, I get this error:

mkdir_hook.c:42:9: error: implicit declaration of function 'mkdir' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
return(mkdir(td, syscall_args));
^
mkdir_hook.c:57:45: error: use of undeclared identifier 'mkdir'
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir;

mkdir(2) requires `#include <sys/stat.h>' and I added it (while it was
not in the code from the book), but nothing changes.

What's still wrong?

After this text, I paste the code and the Makefile (written following
https://docs.freebsd.org/en/books/arch-handbook/driverbasics/).

Bye!

Rocky



=== code: mkdir_hook.c ===

#include <sys/types.h>
#include <sys/param.h> /* defines used in kernel.h */
#include <sys/proc.h>
#include <sys/module.h>
#include <sys/sysent.h>
#include <sys/kernel.h> /* types used in module initialization */
#include <sys/systm.h> /* uprintf */
#include <sys/syscall.h>
#include <sys/sysproto.h>
#include <sys/stat.h> /* mkdir(2) */


/* mkdir system call hook. */
static int
mkdir_hook(struct thread *td, void *syscall_args)
{
struct mkdir_args /* {
char *path;
int mode;
} */ *uap;

uap = (struct mkdir_args *)syscall_args;
char path[255];
size_t done;
int error;
error = copyinstr(uap->path, path, 255, &done);

if (error != 0)
return(error);
/* Print a debug message. */
uprintf("The directory \"%s\" will be created with the following"
" permissions: %o\n", path, uap->mode);

return(mkdir(td, syscall_args));
}

/* The function called at load/unload. */
static int
load(struct module *module, int cmd, void *arg)
{
int error = 0;
switch (cmd) {
case MOD_LOAD:
/* Replace mkdir with mkdir_hook. */
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir_hook;
break;
case MOD_UNLOAD:
/* Change everything back to normal. */
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir;
break;

default:
error = EOPNOTSUPP;
break;
}

return(error);
}

static moduledata_t
mkdir_hook_mod = {
"mkdir_hook", /* module name */
load, /* event handler */
NULL /* extra data */
};

DECLARE_MODULE(mkdir_hook, mkdir_hook_mod, SI_SUB_DRIVERS, SI_ORDER_MIDDLE);



=== Makefile ===

KMOD=mkdir_hook
SRCS=mkdir_hook.c
.include <bsd.kmod.mk>


--
Posted automagically by a mail2news gateway at muc.de e.V.
Please direct questions, flames, donations, etc. to news-***@muc.de

[Yuri]

Post by Rocky Hotas
Hello!
I hope this is the right ML to discuss about this issue. I am trying to
build the example kernel module presented in paragraph 2.1 of
Designing BSD Rootkits: An Introduction to Kernel Hacking
by Joseph Kong
mkdir_hook.c:42:9: error: implicit declaration of function 'mkdir' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
return(mkdir(td, syscall_args));
^
mkdir_hook.c:57:45: error: use of undeclared identifier 'mkdir'
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir;
mkdir(2) requires `#include <sys/stat.h>' and I added it (while it was
not in the code from the book), but nothing changes.

That include is for userland that needs mkdir(2). Your wrapper likely
wants to override the in-kernel sys_mkdir() defined in <sys/sysproto.h>.

Post by Rocky Hotas
What's still wrong?
After this text, I paste the code and the Makefile (written following
https://docs.freebsd.org/en/books/arch-handbook/driverbasics/).
Bye!
Rocky
=== code: mkdir_hook.c ===
#include <sys/types.h>
#include <sys/param.h> /* defines used in kernel.h */
#include <sys/proc.h>
#include <sys/module.h>
#include <sys/sysent.h>
#include <sys/kernel.h> /* types used in module initialization */
#include <sys/systm.h> /* uprintf */
#include <sys/syscall.h>
#include <sys/sysproto.h>
#include <sys/stat.h> /* mkdir(2) */
/* mkdir system call hook. */
static int
mkdir_hook(struct thread *td, void *syscall_args)
{
struct mkdir_args /* {
char *path;
int mode;
} */ *uap;
uap = (struct mkdir_args *)syscall_args;
char path[255];
size_t done;
int error;
error = copyinstr(uap->path, path, 255, &done);
if (error != 0)
return(error);
/* Print a debug message. */
uprintf("The directory \"%s\" will be created with the following"
" permissions: %o\n", path, uap->mode);
return(mkdir(td, syscall_args));
}
/* The function called at load/unload. */
static int
load(struct module *module, int cmd, void *arg)
{
int error = 0;
switch (cmd) {
/* Replace mkdir with mkdir_hook. */
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir_hook;
break;
/* Change everything back to normal. */
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir;
break;
error = EOPNOTSUPP;
break;
}
return(error);
}
static moduledata_t
mkdir_hook_mod = {
"mkdir_hook", /* module name */
load, /* event handler */
NULL /* extra data */
};
DECLARE_MODULE(mkdir_hook, mkdir_hook_mod, SI_SUB_DRIVERS, SI_ORDER_MIDDLE);
=== Makefile ===
KMOD=mkdir_hook
SRCS=mkdir_hook.c
.include <bsd.kmod.mk>

--
Posted automagically by a mail2news gateway at muc.de e.V.
Please direct questions, flames, donations, etc. to news-***@muc.de

[Rocky Hotas]

Sent: Monday, September 18, 2023 at 11:38 PM
Subject: Re: Error while building a kernel module with mkdir(2)
That include is for userland that needs mkdir(2). Your wrapper likely
wants to override the in-kernel sys_mkdir() defined in <sys/sysproto.h>.

Yes, it works this way. Thank you so much!

Now, searching for sys_mkdir, I also found this post:

<https://forums.freebsd.org/threads/link_elf_obj-symbol-mkdir-undefined.71374/>

with the same problem. The book is somewhat outdated.

Are such kernel functions documented somewhere? sys_mkdir(9) does not
exist. The only manpage I could find with `apropos syscall' is
SYSCALL_MODULE(9), but it's not exactly what I was looking for.


Rocky


--
Posted automagically by a mail2news gateway at muc.de e.V.
Please direct questions, flames, donations, etc. to news-***@muc.de

[Dmitry Chagin]

Post by Rocky Hotas
Hello!
I hope this is the right ML to discuss about this issue. I am trying to
build the example kernel module presented in paragraph 2.1 of
Designing BSD Rootkits: An Introduction to Kernel Hacking
by Joseph Kong
mkdir_hook.c:42:9: error: implicit declaration of function 'mkdir' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
return(mkdir(td, syscall_args));
^
mkdir_hook.c:57:45: error: use of undeclared identifier 'mkdir'
sysent[SYS_mkdir].sy_call = (sy_call_t *)mkdir;
mkdir(2) requires `#include <sys/stat.h>' and I added it (while it was
not in the code from the book), but nothing changes.
What's still wrong?

No more mkdir, look for kern_mkdirat at syscallsubr.h




--
Posted automagically by a mail2news gateway at muc.de e.V.
Please direct questions, flames, donations, etc. to news-***@muc.de