Re: [PATCH 01/19] vfs: syscall: Add fsinfo() to query filesystem information [ver #16]

From: Jann Horn <jannh@google.com>
To: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	raven@themaw.net, Miklos Szeredi <mszeredi@redhat.com>,
	Christian Brauner <christian@brauner.io>,
	Linux API <linux-api@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 01/19] vfs: syscall: Add fsinfo() to query filesystem information [ver #16]
Date: Thu, 20 Feb 2020 15:54:25 +0100	[thread overview]
Message-ID: <CAG48ez00KA3tjeccDCeqmgHyppTLEr+UkrB=QaQ-FX-cTY3aCA@mail.gmail.com> (raw)
In-Reply-To: <584179.1582196636@warthog.procyon.org.uk>

On Thu, Feb 20, 2020 at 12:04 PM David Howells <dhowells@redhat.com> wrote:
> Jann Horn <jannh@google.com> wrote:
>
> > > +int fsinfo_string(const char *s, struct fsinfo_context *ctx)
> > ...
> > Please add a check here to ensure that "ret" actually fits into the
> > buffer (and use WARN_ON() if you think the check should never fire).
> > Otherwise I think this is too fragile.
>
> How about:
>
>         int fsinfo_string(const char *s, struct fsinfo_context *ctx)
>         {
>                 unsigned int len;
>                 char *p = ctx->buffer;
>                 int ret = 0;
>                 if (s) {
>                         len = strlen(s);
>                         if (len > ctx->buf_size - 1)
>                                 len = ctx->buf_size;
>                         if (!ctx->want_size_only) {
>                                 memcpy(p, s, len);
>                                 p[len] = 0;

I think this is off-by-one? If len was too big, it is set to
ctx->buf_size, so in that case this effectively becomes
`ctx->buffer[ctx->buf_size] = 0`, which is one byte out of bounds,
right?

Maybe use something like `len = min_t(size_t, strlen(s), ctx->buf_size-1)` ?

Looks good apart from that, I think.

>                         }
>                         ret = len;
>                 }
>                 return ret;
>         }
[...]
> > > +       return ctx->usage;
> >
> > It is kind of weird that you have to return the ctx->usage everywhere
> > even though the caller already has ctx...
>
> At this point, it's only used and returned by fsinfo_attributes() and really
> is only for the use of the attribute getter function.
>
> I could, I suppose, return the amount of data in ctx->usage and then preset it
> for VSTRUCT-type objects.  Unfortunately, I can't make the getter return void
> since it might have to return an error.

Yeah, then you'd be passing around the error separately from the
length... I don't know whether that'd make things better or worse.

[...]
> > > +struct fsinfo_attribute {
> > > +       unsigned int            attr_id;        /* The ID of the attribute */
> > > +       enum fsinfo_value_type  type:8;         /* The type of the attribute's value(s) */
> > > +       unsigned int            flags:8;
> > > +       unsigned int            size:16;        /* - Value size (FSINFO_STRUCT) */
> > > +       unsigned int            element_size:16; /* - Element size (FSINFO_LIST) */
> > > +       int (*get)(struct path *path, struct fsinfo_context *params);
> > > +};
> >
> > Why the bitfields? It doesn't look like that's going to help you much,
> > you'll just end up with 6 bytes of holes on x86-64:
>
> Expanding them to non-bitfields will require an extra 10 bytes, making the
> struct 8 bytes bigger with 4 bytes of padding.  I can do that if you'd rather.

Wouldn't this still have the same total size?

struct fsinfo_attribute {
  unsigned int attr_id;        /* 0x0-0x3 */
  enum fsinfo_value_type type; /* 0x4-0x7 */
  u8 flags;                    /* 0x8-0x8 */
  /* 1-byte hole */
  u16 size;                    /* 0xa-0xb */
  u16 element_size;            /* 0xc-0xd */
  /* 2-byte hole */
  int (*get)(...);             /* 0x10-0x18 */
};

But it's not like I really care about this detail all that much, feel
free to leave it as-is.