Commit 7f4d0069 authored by Andrew Boie's avatar Andrew Boie Committed by Andrew Boie
Browse files

kernel: fix errno access for user mode 



The errno "variable" is required to be thread-specific.
It gets defined to a macro which dereferences a pointer
returned by a kernel function.

In user mode, we cannot simply read/write the thread struct.
We do not have thread-local storage mechanism, so for now
use the lowest address of the thread stack to store this
value, since this is guaranteed to be read/writable by
a user thread.

The downside of this approach is potential stack corruption
if the stack pointer goes down this far but does not exceed
the location, since a fault won't be generated in this case.

Signed-off-by: default avatarAndrew Boie <andrew.p.boie@intel.com>
parent 89f87ec5

include/kernel.h

+8 −0
Original line number Diff line number Diff line
@@ -538,9 +538,17 @@ struct k_thread { 
#endif



#ifdef CONFIG_ERRNO

#ifdef CONFIG_USERSPACE

	/* Set to the lowest area in the thread stack since this needs to

	 * be directly read/writable by user mode. Not ideal, but best we

	 * can do until we have thread-local storage

	 */

	int *errno_location;

#else

	/** per-thread errno variable */

	int errno_var;

#endif

#endif



#if defined(CONFIG_THREAD_STACK_INFO)

	/** Stack Info */

include/misc/errno_private.h

0 → 100644
+36 −0
Original line number Diff line number Diff line 
/*

 * Copyright (c) 2018 Intel Corporation.

 *

 * SPDX-License-Identifier: Apache-2.0

 */



#ifndef _MISC_ERRNO_PRIVATE_H_

#define _MISC_ERRNO_PRIVATE_H_



#include <toolchain.h>



#ifdef __cplusplus

extern "C" {

#endif



/* NOTE: located here to avoid include dependency loops between errno.h

 * and kernel.h

 */



/**

 * return a pointer to a memory location containing errno

 *

 * errno is thread-specific, and can't just be a global. This pointer

 * is guaranteed to be read/writable from user mode.

 *

 * @return Memory location of errno data for current thread

 */

__syscall int *z_errno(void);



#ifdef __cplusplus

}

#endif



#include <syscalls/errno_private.h>



#endif /* _MISC_ERRNO_PRIVATE_H */

kernel/errno.c

+15 −2
Original line number Diff line number Diff line
@@ -13,6 +13,7 @@ 
 */



#include <kernel_structs.h>

#include <syscall_handler.h>



/*

 * Define _k_neg_eagain for use in assembly files as errno.h is
@@ -22,8 +23,20 @@ 
const int _k_neg_eagain = -EAGAIN;



#ifdef CONFIG_ERRNO

int *__errno(void)

#ifdef CONFIG_USERSPACE

int *_impl_z_errno(void)

{

	/* Initialized to the lowest address in the stack so the thread can

	 * directly read/write it

	 */

	return _current->errno_location;

}



Z_SYSCALL_HANDLER0_SIMPLE(z_errno);

#else

int *_impl_z_errno(void)

{

	return &_current->errno_var;

}

#endif
 
#endif /* CONFIG_USERSPACE */

#endif /* CONFIG_ERRNO */

kernel/thread.c

+1 −0
Original line number Diff line number Diff line
@@ -312,6 +312,7 @@ void _setup_new_thread(struct k_thread *new_thread, 
	_k_object_init(new_thread);

	_k_object_init(stack);

	new_thread->stack_obj = stack;

	new_thread->errno_location = (int *)K_THREAD_STACK_BUFFER(stack);



	/* Any given thread has access to itself */

	k_object_access_grant(new_thread, new_thread);

lib/libc/minimal/include/errno.h

+3 −3
Original line number Diff line number Diff line
@@ -17,13 +17,13 @@ 
#ifndef __INCerrnoh

#define __INCerrnoh



#include <misc/errno_private.h>



#ifdef __cplusplus

extern "C" {

#endif





extern int *__errno(void);

#define errno (*__errno())

#define errno (*z_errno())



/*

 * POSIX Error codes

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