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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 26 Sep 2023 16:33:50 +0200
Message-ID: <CAJfpegvKECAFNhWYKfGbSWVX8pycQxsHnCr6KSqrQrR+u77yAg@mail.gmail.com>
Subject: Re: [RFC PATCH 2/3] add statmnt(2) syscall
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, linux-man@vger.kernel.org,
        linux-security-module@vger.kernel.org, Karel Zak <kzak@redhat.com>,
        Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Amir Goldstein <amir73il@gmail.com>
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On Tue, 26 Sept 2023 at 16:19, Florian Weimer <fweimer@redhat.com> wrote:

> getdents gets away with this buffer size because applications can copy
> out all the data from struct dirent if they need long-term storage.
> They have to do that because the usual readdir interface overwrites the
> buffer, potentially at the next readdir call.  This means the buffer
> size does not introduce an amount of memory fragmention that is
> dependent on the directory size.
>
> With an opaque, pointer-carrying struct, copying out the data is not
> possible in a generic fashion.  Only the parts that the application
> knows about can be copied out.  So I think it's desirable to have a
> fairly exact allocation.

Okay, so let's add a 'size' field to the struct, which is set to the
size used (as opposed to the size of the buffer).   That should solve
copying without wasting a single byte of memory.

Otherwise the format is fully copyable, since the strings are denoted
with an offset, which doesn't change after the buffer is copied.

Thanks,
Miklos