Re: [PATCH v1 1/2] io_uring,audit: audit IORING_OP_FADVISE but not IORING_OP_MADVISE

[Steve Grubb]

On Friday, January 27, 2023 5:57:30 PM EST Paul Moore wrote:
> On Fri, Jan 27, 2023 at 5:45 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
> > On 1/27/23 3:35?PM, Paul Moore wrote:
> > > On Fri, Jan 27, 2023 at 12:24 PM Richard Guy Briggs <rgb@xxxxxxxxxx> 
wrote:
> > >> Since FADVISE can truncate files and MADVISE operates on memory,
> > >> reverse
> > >> the audit_skip tags.
> > >> 
> > >> Fixes: 5bd2182d58e9 ("audit,io_uring,io-wq: add some basic audit
> > >> support to io_uring") Signed-off-by: Richard Guy Briggs
> > >> <rgb@xxxxxxxxxx>
> > >> ---
> > >> 
> > >>  io_uring/opdef.c | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >> 
> > >> diff --git a/io_uring/opdef.c b/io_uring/opdef.c
> > >> index 3aa0d65c50e3..a2bf53b4a38a 100644
> > >> --- a/io_uring/opdef.c
> > >> +++ b/io_uring/opdef.c
> > >> @@ -306,12 +306,12 @@ const struct io_op_def io_op_defs[] = {
> > >> 
> > >>         },
> > >>         [IORING_OP_FADVISE] = {
> > >>         
> > >>                 .needs_file             = 1,
> > >> 
> > >> -               .audit_skip             = 1,
> > >> 
> > >>                 .name                   = "FADVISE",
> > >>                 .prep                   = io_fadvise_prep,
> > >>                 .issue                  = io_fadvise,
> > >>         
> > >>         },
> > > 
> > > I've never used posix_fadvise() or the associated fadvise64*()
> > > syscalls, but from quickly reading the manpages and the
> > > generic_fadvise() function in the kernel I'm missing where the fadvise
> > > family of functions could be used to truncate a file, can you show me
> > > where this happens?  The closest I can see is the manipulation of the
> > > page cache, but that shouldn't actually modify the file ... right?
> > 
> > Yeah, honestly not sure where that came from. Maybe it's being mixed up
> > with fallocate?
> 
> That was my thought too when I was looking at it.

Oh. Yeah. fallocate is the one that truncates. fadvise can be skipped.

-Steve

> > All fadvise (or madvise, for that matter) does is
> > provide hints on the caching or access pattern. On second thought, both
> > of these should be able to set audit_skip as far as I can tell.
> 
> Agreed on the fadvise side, and probably the madvise side too,
> although the latter has more options/code to sift through so I'm
> curious to hear what analysis Richard has done on that one.