Re: madvise/fadvise 32-bit length

[Jens Axboe <axboe@kernel.dk>]

On 6/1/24 8:19 AM, Jens Axboe wrote:
> On 6/1/24 3:43 AM, Stefan wrote:
>> io_uring uses the __u32 len field in order to pass the length to
>> madvise and fadvise, but these calls use an off_t, which is 64bit on
>> 64bit platforms.
>>
>> When using liburing, the length is silently truncated to 32bits (so
>> 8GB length would become zero, which has a different meaning of "until
>> the end of the file" for fadvise).
>>
>> If my understanding is correct, we could fix this by introducing new
>> operations MADVISE64 and FADVISE64, which use the addr3 field instead
>> of the length field for length.
> 
> We probably just want to introduce a flag and ensure that older stable
> kernels check it, and then use a 64-bit field for it when the flag is
> set.

I think this should do it on the kernel side, as we already check these
fields and return -EINVAL as needed. Should also be trivial to backport.
Totally untested... Might want a FEAT flag for this, or something where
it's detectable, to make the liburing change straight forward.


diff --git a/io_uring/advise.c b/io_uring/advise.c
index 7085804c513c..cb7b881665e5 100644
--- a/io_uring/advise.c
+++ b/io_uring/advise.c
@@ -17,14 +17,14 @@
 struct io_fadvise {
 	struct file			*file;
 	u64				offset;
-	u32				len;
+	u64				len;
 	u32				advice;
 };
 
 struct io_madvise {
 	struct file			*file;
 	u64				addr;
-	u32				len;
+	u64				len;
 	u32				advice;
 };
 
@@ -33,11 +33,13 @@ int io_madvise_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 #if defined(CONFIG_ADVISE_SYSCALLS) && defined(CONFIG_MMU)
 	struct io_madvise *ma = io_kiocb_to_cmd(req, struct io_madvise);
 
-	if (sqe->buf_index || sqe->off || sqe->splice_fd_in)
+	if (sqe->buf_index || sqe->splice_fd_in)
 		return -EINVAL;
 
 	ma->addr = READ_ONCE(sqe->addr);
-	ma->len = READ_ONCE(sqe->len);
+	ma->len = READ_ONCE(sqe->off);
+	if (!ma->len)
+		ma->len = READ_ONCE(sqe->len);
 	ma->advice = READ_ONCE(sqe->fadvise_advice);
 	req->flags |= REQ_F_FORCE_ASYNC;
 	return 0;
@@ -78,11 +80,13 @@ int io_fadvise_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_fadvise *fa = io_kiocb_to_cmd(req, struct io_fadvise);
 
-	if (sqe->buf_index || sqe->addr || sqe->splice_fd_in)
+	if (sqe->buf_index || sqe->splice_fd_in)
 		return -EINVAL;
 
 	fa->offset = READ_ONCE(sqe->off);
-	fa->len = READ_ONCE(sqe->len);
+	fa->len = READ_ONCE(sqe->addr);
+	if (!fa->len)
+		fa->len = READ_ONCE(sqe->len);
 	fa->advice = READ_ONCE(sqe->fadvise_advice);
 	if (io_fadvise_force_async(fa))
 		req->flags |= REQ_F_FORCE_ASYNC;

-- 
Jens Axboe