Re: [Qemu-devel] [PATCH] linux-user: Use *at functions instead of caching interp_prefix contents

[Richard Henderson]

On 01/12/2017 02:35 AM, Peter Maydell wrote:
> On 12 January 2017 at 04:05, Richard Henderson <address@hidden> wrote:
> > If the interp_prefix is a complete chroot, it may have a *lot* of files.
> > Setting up the cache for this is quite expensive.  Instead, use the *at
> > versions of various syscalls to attempt the operation in the prefix.
>
> Presumably this also fixes the completely broken handling of
> symlinks in the interp_prefix tree?

Presumably.

> Unfortunately patch will break bsd-user as it stands,
> because bsd-user also calls init_paths. There's also a
> usage in tests/tcg/.

Oops, yes.

> Awkward problem: I suspect you'll find this breaks a few
> guest programs which don't like the fact that there's now
> an open fd which doesn't belong to the guest. (For instance
> IIRC some LTP test programs do "check that the rlimit on
> number of open files hits after opening exactly N files".)

Hum.  Probably.  I don't see how I can avoid that.

> Incidentally it's a shame C doesn't make it easier to
> abstract out the repeated pattern
>
>         switch (PATHNAME[0]) {
>         case '/':
>             ret = SOMEFN(interp_dirfd, PATHNAME + 1, STUFF);
>             if (ret == 0 || errno != ENOENT) {
>                 break;
>             }
>             /* fallthru */
>         default:
>             ret = SOMEFN(FD, PATHNAME, STUFF);
>             break;
>         }
>
> (and the variant which uses a 2nd function).
>
> A macro would get awkwardly heavily parameterised, I think.

I did briefly play with such a macro, and I found it to be uglier than just 
replicating the pattern.


r~