[RFC][PATCH][bugfix] more checks for negative f_pos handling (WasRe: Question: how to handle too big f_pos

[KAMEZAWA Hiroyuki]

The problem:
> I'm writing a patch against /dev/kmem...I found a problem.
> 
> /dev/kmem (and /proc/<pid>/mem) puts virtual addres to f->f_pos.
> 
> but f->f_pos is always negative and rw_verify_ara() returns -EINVAL always.

Changed CC: List. 

This is a trial to consider how to fix negative f_pos problem shown in above.

Hmm, even after this patch, x86's vsyscall area is not readable.
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0  [vsyscall]
But maybe no problems. (now, it cannot be read, anyway.)

I tested /dev/kmem on x86-64 and this works fine. I added a fix for
/proc/<pid>/mem because I know ia64's hugetlbe area is not readable
via /proc/<pid>/mem. (But I'm not sure other 64bit arch has this
kind of problems in /proc/<pid>/mem)

==
From: KAMEZAWA Hiruyoki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Modifying rw_verify_area()'s negative f_pos check.

Now, rw_verify_area() has this check
   if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
		return -EINVAL

And access to special files as /dev/mem,kmem, /proc/<pid>/mem
returns unexpected -EINVAL.
(For example, ia64 maps hugetlb at 0x8000000000000000- region)

This patch tries to make range check more precise by using
llseek ops defined per special files.

Signed-off-by: KAMEZAWA Hiruyoki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 fs/proc/base.c  |   22 +++++++++++++++++-----
 fs/read_write.c |   39 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 54 insertions(+), 7 deletions(-)

Index: mmotm-2.6.31-Sep14/fs/read_write.c
===================================================================
--- mmotm-2.6.31-Sep14.orig/fs/read_write.c
+++ mmotm-2.6.31-Sep14/fs/read_write.c
@@ -205,6 +205,37 @@ bad:
 }
 #endif
 
+static int
+__verify_negative_pos_range(struct file *file, loff_t pos, size_t count)
+{
+	unsigned long long upos, end;
+	loff_t ret;
+
+	/* disallow overflow */
+	upos = (unsigned long long)pos;
+	end = upos + count;
+	if (end < pos)
+		return -EOVERFLOW;
+	/*
+	 * Sanity check...subsystem has to provide llseek for handle big pos.
+	 * Subsystem's llseek should verify f_pos's value comaparing with its
+	 * max file size.
+	 * Note1: generic file ops' llseek cannot handle negative pos.
+	 * Note2: should we take care of pos == -EINVAL ?
+	 * Note3: we check flags and ops here for avoiding taking locks in.
+	 * default_lseek.
+	 */
+	ret = -EINVAL;
+	if ((file->f_mode & FMODE_LSEEK) &&
+	    (file->f_op && file->f_op->llseek)) {
+		ret = vfs_llseek(file, 0, SEEK_CUR);
+		if (ret == pos)
+			return 0;
+	}
+
+	return (int)ret;
+}
+
 /*
  * rw_verify_area doesn't like huge counts. We limit
  * them to something that fits in "int" so that others
@@ -222,8 +253,12 @@ int rw_verify_area(int read_write, struc
 	if (unlikely((ssize_t) count < 0))
 		return retval;
 	pos = *ppos;
-	if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
-		return retval;
+	if (unlikely((pos < 0) || (loff_t) (pos + count) < 0)) {
+		/* some files requires special care */
+		retval = __verify_negative_pos_range(file, pos, count);
+		if (retval)
+			return retval;
+	}
 
 	if (unlikely(inode->i_flock && mandatory_lock(inode))) {
 		retval = locks_mandatory_area(
Index: mmotm-2.6.31-Sep14/fs/proc/base.c
===================================================================
--- mmotm-2.6.31-Sep14.orig/fs/proc/base.c
+++ mmotm-2.6.31-Sep14/fs/proc/base.c
@@ -903,18 +903,30 @@ out_no_task:
 
 loff_t mem_lseek(struct file *file, loff_t offset, int orig)
 {
+	struct task_struct *task = get_proc_task(file->f_path.dentry->d_inode);
+	unsigned long long new_offset = -EINVAL;
+
+	if (!task) /* lseek's spec doesn't allow -ESRCH but... */
+		return -ESRCH;
+
 	switch (orig) {
 	case 0:
-		file->f_pos = offset;
+		new_offset = offset;
 		break;
 	case 1:
-		file->f_pos += offset;
+		new_offset = (unsigned long long)f->f_pos + offset;
 		break;
 	default:
-		return -EINVAL;
+		new_offset = -EINVAL;
+		break;
 	}
-	force_successful_syscall_return();
-	return file->f_pos;
+	if (new_offset < (unsigned long long)TASK_SIZE_OF(task)) {
+		file->f_pos = (loff_t)new_offset;
+		force_successful_syscall_return();
+	} else
+		new_offset = -EINVAL;
+	put_task_struct(task);
+	return (loff_t)new_offset;
 }
 
 static const struct file_operations proc_mem_operations = {

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