unix: fix int overflow when copying large files

I was getting some weird results when copying a 5GB file on Alpine
Linux on Raspberry Pi. Out of the 5GB only 1.1GB would get copied and
the process would finish without an error. After some digging I found
out that there's a problem that some data types have smaller number of
bytes on Alpine Linux on Raspberry Pi than on other platforms
apparently.

When getting the size of the file in bytes, stat holds the size in
off_t data type, like this:

    struct stat {
        ...
        off_t     st_size;    /* total size, in bytes */
        ...
    };

In my case, off_t has 8 bytes which is enough to hold a value up to
some exabytes. The problem is that it gets assigned to bytes_to_send
variable, which is size_t. In my case, size_t is only 4 bytes, which
is only good for about 4GB. If the file is any larger, there's an
overflow when assigning it from stat to bytes_to_send. That's easy
to fix, I just changed the data type of bytes_to_send to off_t.
However there's more.

The other 2 variables - in_offset and bytes_written also have to be
able to hold the size of the entire file, therefore it makes sense to
change them to off_t as well.

The last problem is that bytes_to_send is passed down to
uv_fs_sendfile() which converts it to size_t again. I could go and
change the types everywhere across the whole codebase to off_t but
that could break other things, so it seams a bit too much. A much
better solution is to have a proxy variable bytes_chunk that is
size_t type and copy as much bytes as possible at a time that can
fit into size_t. That way it will work the same as before on other
platforms, where size_t is more than 4 bytes.

PR-URL: https://github.com/libuv/libuv/pull/2758
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

0 files changed, 0 insertions, 0 deletions