Page Fault on calling a (garbage) function pointer
Posted: Fri Mar 07, 2014 7:59 pm
Prefix: I don't know that this forum is exactly appropriate for this question, but I'll try anyway.
My kernels VFS code is page faulting on attempting to call the handler for reading the /dev/null device. It is defined as a function pointer to the read_null function in device_null_create(). The page fault occurs immediately when read_null is called from the vfs_node_t object held in the filesystem tree. The cause of this is an invalid pointer to 0xF000FF53, which is a garbage address (at least, I believe it is; my kernel at this point had only allocated to about 0x2F4000 of usable memory, as it is a lower half kernel).
My question is: What's causing my kernel to call that address instead of the proper address of my function?
Single-stepping in gdb has revealed little (or maybe it's because I don't use gdb often), and everything seems ("seems") okay until read_null is called. I have posted some relevant sources and output as well, and attempted to narrow it down to related and suspected functions.
main.c
vfs.c
device_null.c
Kernel Log (Thank god I had the insight to print all output to the serial console (stdio), so it was an easy copy & paste)
I've tried to work this bug out, but it is either beyond my capabilities (I am still learning; but I do have at least half a head on my shoulders), or it's so simplistic it is flying over my head. If you need anymore just ask, but full up to date source code is in my github in the dev branch for convenience. I've also tried to ask this question intelligently, so if I made any errors in asking, could you tell me? I wish to improve.
Thanks,
Lionel
My kernels VFS code is page faulting on attempting to call the handler for reading the /dev/null device. It is defined as a function pointer to the read_null function in device_null_create(). The page fault occurs immediately when read_null is called from the vfs_node_t object held in the filesystem tree. The cause of this is an invalid pointer to 0xF000FF53, which is a garbage address (at least, I believe it is; my kernel at this point had only allocated to about 0x2F4000 of usable memory, as it is a lower half kernel).
My question is: What's causing my kernel to call that address instead of the proper address of my function?
Single-stepping in gdb has revealed little (or maybe it's because I don't use gdb often), and everything seems ("seems") okay until read_null is called. I have posted some relevant sources and output as well, and attempted to narrow it down to related and suspected functions.
main.c
Code: Select all
vfs_mount("/dev/null", device_null_create());
vfs_print_tree_node(vfs_tree->root,0);
printf("Reading from /dev/null\n");
vfs_node_t *null = kopen("/dev/null", 0);
printf("null returns:%d\n",read_vfs(null,0,0,0));
Code: Select all
uint32_t read_vfs(vfs_node_t *node, uint32_t offset, uint32_t size, uint8_t *buffer)
{
if (node->read) {
printf("read_vfs: node->read = 0x%X\n",node->read);
uint32_t ret = node->read(node, offset, size, buffer);
return ret;
} else {
return 0;
}
}
Code: Select all
uint32_t read_null(vfs_node_t *node, uint32_t offset, uint32_t size, uint8_t *buffer) {
return 0;
}
...
vfs_node_t *device_null_create()
{
vfs_node_t * fnode = malloc(sizeof(vfs_node_t));
memset(fnode, 0x00, sizeof(vfs_node_t));
fnode->inode = 0;
strcpy(fnode->name, "null");
fnode->uid = 0;
fnode->gid = 0;
fnode->flags = VFS_CHARDEVICE;
fnode->read = read_null;
fnode->write = write_null;
fnode->open = open_null;
fnode->close = close_null;
fnode->readdir = NULL;
fnode->finddir = NULL;
return fnode;
}
Code: Select all
[x86]:Loading x86 components...
[CPU]:GDT Setup
[CPU]:IDT Setup
[IRQ]:Registered irq handler for 32 (IRQ0) at 0x10BD10
[IRQ]:Registered irq handler for 14 at 0x109EF0
[MEM]:Initialising and populating memory...
[MEM]:Marked 0x2F4000 (3096576) frames as dirty
[MEM]:Allocating Kernel Reserved Area...
[MEM]:Creating heap...!
[MEM]:Done allocating initial memory!
[x86]:Done starting hardware!
[KERN]:CoreLibs initialising...
[KERN]:Running Debug Kernel! Some things might not work properly!
[IO]:Verifiying timer / interrupts (waiting 10 ticks)
[KERN]:Finished initialising CoreLibs!
[VIDEO]:BGA unsupported, setting terminal as output
[VFS]:Starting VFS...
sbrk: allocating 1 pages to cover 0x1000 bytes
[root] -> (empty)
[VFS]:Mounting devices...
[VFS]:Searching for dev
[VFS]:Did not find dev, making it.
[VFS]:Searching for null
[VFS]:Did not find null, making it.
[root] -> (empty)
dev -> (empty)
null -> 0x2f4091 (null)
Reading from /dev/null
read_vfs: node->read = 0xF000FF53
[PF]:Page fault!
[PF]:Addr:0xF000FF53. present 1 rw 0 us 0 res 0 id 0
[SYS]:Encountered interrupt 14 (Page Fault)!
[KERN]:Halting!
Registers:
| eax 0x22; ebx 0x0; ecx 0xF000FF53; edx 0x0
| ??? 0x104AB8; ebp 0x104B08; err 0x0; efl 0x202
| usp 0x105E31; eip 0x104B08; esi 0x0; edi 0x0
| cs 0x8; ds 0x10; es 0x10; fs 0x10
| gs 0x10
Thanks,
Lionel
