lseek(fd, addr, SEEK_DATA) adjust file offset to the start address of next VMA,
or to addr if this address is allocated.
lseek(fd, addr, SEEK_HOLE) adjust file offset to the end address of VMA which
addr belongs to, or to addr itself if there is hole.
This way SEEK_HOLE reports a virtual zero-length hole between each contiguous
VMAs. This hack seems completely legit and allows to simplify implementation
(there is no function for finding next hole in VMAs' tree, walking along
->vm_next might be expensive) This also gives more information about layout.
Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
---
I have no practical use for this, just found this interesting.
---
fs/proc/base.c | 31 +++++++++++++++++++++++++++++--
1 file changed, 29 insertions(+), 2 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 2d696b0..aba4b47 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -769,13 +769,40 @@ static ssize_t mem_write(struct file *file, const char __user *buf,
loff_t mem_lseek(struct file *file, loff_t offset, int orig)
{
+ struct mm_struct *mm = file->private_data;
+ struct vm_area_struct *vma;
+
switch (orig) {
- case 0:
+ case SEEK_SET:
file->f_pos = offset;
break;
- case 1:
+ case SEEK_CUR:
file->f_pos += offset;
break;
+ case SEEK_DATA:
+ case SEEK_HOLE:
+ if (!mm || !atomic_inc_not_zero(&mm->mm_users))
+ return -ENXIO;
+ down_read(&mm->mmap_sem);
+ vma = find_vma(mm, offset);
+ if (vma) {
+ if (orig == SEEK_DATA) {
+ if (offset >= vma->vm_start)
+ file->f_pos = offset;
+ else
+ file->f_pos = vma->vm_start;
+ } else {
+ if (offset < vma->vm_start)
+ file->f_pos = offset;
+ else
+ file->f_pos = vma->vm_end;
+ }
+ }
+ up_read(&mm->mmap_sem);
+ mmput(mm);
+ if (!vma)
+ return -ENXIO;
+ break;
default:
return -EINVAL;
}
--
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