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[iov_iter] new privimitive: iov_iter_revert() 

opposite to iov_iter_advance(); the caller is responsible for never
using it to move back past the initial position.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2017-02-17 18:42:24 -05:00
parent c02ed2e75e
commit 27c0e3748e
2 changed files with 68 additions and 1 deletions
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6
include/linux/uio.h
View File

@ -39,7 +39,10 @@ struct iov_iter {
};
union {
unsigned long nr_segs;
int idx;
struct {
int idx;
int start_idx;
};
};
};
@ -81,6 +84,7 @@ unsigned long iov_shorten(struct iovec *iov, unsigned long nr_segs, size_t to);
size_t iov_iter_copy_from_user_atomic(struct page *page,
struct iov_iter *i, unsigned long offset, size_t bytes);
void iov_iter_advance(struct iov_iter *i, size_t bytes);
void iov_iter_revert(struct iov_iter *i, size_t bytes);
int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes);
size_t iov_iter_single_seg_count(const struct iov_iter *i);
size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,

63
lib/iov_iter.c
View File

@ -786,6 +786,68 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
}
EXPORT_SYMBOL(iov_iter_advance);
void iov_iter_revert(struct iov_iter *i, size_t unroll)
{
if (!unroll)
return;
i->count += unroll;
if (unlikely(i->type & ITER_PIPE)) {
struct pipe_inode_info *pipe = i->pipe;
int idx = i->idx;
size_t off = i->iov_offset;
while (1) {
size_t n = off - pipe->bufs[idx].offset;
if (unroll < n) {
off -= (n - unroll);
break;
}
unroll -= n;
if (!unroll && idx == i->start_idx) {
off = 0;
break;
}
if (!idx--)
idx = pipe->buffers - 1;
off = pipe->bufs[idx].offset + pipe->bufs[idx].len;
}
i->iov_offset = off;
i->idx = idx;
pipe_truncate(i);
return;
}
if (unroll <= i->iov_offset) {
i->iov_offset -= unroll;
return;
}
unroll -= i->iov_offset;
if (i->type & ITER_BVEC) {
const struct bio_vec *bvec = i->bvec;
while (1) {
size_t n = (--bvec)->bv_len;
i->nr_segs++;
if (unroll <= n) {
i->bvec = bvec;
i->iov_offset = n - unroll;
return;
}
unroll -= n;
}
} else { /* same logics for iovec and kvec */
const struct iovec *iov = i->iov;
while (1) {
size_t n = (--iov)->iov_len;
i->nr_segs++;
if (unroll <= n) {
i->iov = iov;
i->iov_offset = n - unroll;
return;
}
unroll -= n;
}
}
}
EXPORT_SYMBOL(iov_iter_revert);
/*
* Return the count of just the current iov_iter segment.
*/
@ -839,6 +901,7 @@ void iov_iter_pipe(struct iov_iter *i, int direction,
i->idx = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);
i->iov_offset = 0;
i->count = count;
i->start_idx = i->idx;
}
EXPORT_SYMBOL(iov_iter_pipe);