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Full Discussion: can a linux kernel module call libc functions?
Top Forums Programming can a linux kernel module call libc functions? Post 302573888 by otheus on Wednesday 16th of November 2011 03:29:00 AM
Old 11-16-2011
Quote:
But, what makes the kernel can not call syscalls which inside itself?
(If you received mail of this post, it was premature. Disregards)

Linux (AFAIK) does not support a 're-entrant' model. Once you're inside the kernel, you can't thread a new process, or go again into the kernel (you're already there). Allowing a module to do a syscall would make things incredibly complicated and would more than often result in a deadlock or worse -- a infinite recursive loop, filling up the stack.

Some UNIX's (Solaris 2.5+) are re-entrant and theoretically could support a kernel module making a syscall, but again, the problems are similar - extreme care has to be taken to ensure that bad things dont happen.

If you want to have a kernel module do some nifty things, the common practice is to have it communicate with a userland process via a pipe or ioctl. That is, make your module act as a dummy device driver which fills the dummy device with data that it wants to send to the userland process. The userland process is then blocking-read on this device and acts appropriately when the data is available.

There is also the issue of calling symantics. On most devices (ie, CPUs), invoking a system call involves setting registers and triggering an interrupt, which the kernel then handles. If your code is running in the Kernel, this interrupt has to be masked to prevent another system call from interrupting the currently executing one. (Unless you're a re-entrant kernel like Solaris; then your code has to be re-entrant too). In other instances, the system call entry point is through a specific CPU instruction -- one which requires a mode bit to indicate the currently executing process is in "userland" mode. Once the control is handed to the kernel, the kernel sets that bit accordingly, so once again, the CPU wont let a kernel thread invoke a kernel thread.
Last edited by otheus; 11-16-2011 at 04:40 AM.. Reason: first response was already answered by corona
 

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LEARN ABOUT CENTOS

xpt

XPT(4)							   BSD Kernel Interfaces Manual 						    XPT(4)

NAME

     xpt -- CAM transport layer interface

SYNOPSIS

     None.

DESCRIPTION

     The xpt driver provides a way for userland applications to issue certain CAM CCBs to the kernel.

     Since the xpt driver allows direct access to the CAM subsystem, system administrators should exercise caution when granting access to this
     driver.  If used improperly, this driver can allow userland applications to crash a machine or cause data loss.

KERNEL CONFIGURATION

     There is no kernel configuration required for the xpt driver.  It is enabled when SCSI support is enabled in the kernel.  There is one
     instance of the xpt driver per CAM transport layer instance.  Since there is currently only one CAM transport layer, there will only be one
     instance of this driver.

IOCTLS

     CAMIOCOMMAND    This ioctl takes certain kinds of CAM CCBs and passes them through to the CAM transport layer for action.	Only the following
		     CCB types are supported:

		     XPT_SCAN_BUS
		     XPT_RESET_BUS
		     XPT_SCAN_LUN
		     XPT_ENG_INQ
		     XPT_ENG_EXEC
		     XPT_DEBUG
		     XPT_DEV_MATCH
		     XPT_PATH_INQ

		     The above CCBs are the only ones supported since it makes more sense to send them through a generic passthrough device rather
		     than a passthrough device tied to a particular underlying SCSI device.

     CAMGETPASSTHRU  This ioctl takes an XPT_GDEVLIST CCB, and returns the passthrough device corresponding to the device in question.

FILES

     /dev/xpt0	Character device node for the xpt driver.

DIAGNOSTICS

     None.

SEE ALSO

     cam(3), cam_cdbparse(3), pass(4), camcontrol(8)

HISTORY

     The CAM transport layer driver first appeared in FreeBSD 3.0.

AUTHORS

     Kenneth Merry <ken@FreeBSD.org>

BSD
								 October 10, 1998							       BSD
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