linux-kernel - Re: [PATCH v12 11/12] open: openat2(2) syscall

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Date:   Sat, 7 Sep 2019 10:42:23 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jeff Layton <jlayton@...nel.org>, Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Aleksa Sarai <asarai@...e.de>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        alpha <linux-alpha@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
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        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
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        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v12 11/12] open: openat2(2) syscall



> On Sep 7, 2019, at 9:58 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Sat, Sep 7, 2019 at 5:40 AM Jeff Layton <jlayton@...nel.org> wrote:
>> 
>> After thinking about this a bit, I wonder if we might be better served
>> with a new set of OA2_* flags instead of repurposing the O_* flags?
> 
> I'd hate to have yet _another_ set of translation functions, and
> another chance of people just getting it wrong either in user space or
> the kernel.
> 
> So no. Let's not make another set of flags that has no sane way to
> have type-safety to avoid more confusion.
> 
> The new flags that _only_ work with openat2() might be named with a
> prefix/suffix to mark that, but I'm not sure it's a huge deal.
> 
>            

I agree with the philosophy, but I think it doesn’t apply in this case.  Here are the flags:

O_RDONLY, O_WRONLY, O_RDWR: not even a proper bitmask. The kernel already has the FMODE_ bits to make this make sense. How about we make the openat2 permission bits consistent with the internal representation and let the O_ permission bits remain as an awful translation.  The kernel already translates like this, and it already sucks.

O_CREAT, O_TMPFILE, O_NOCTTY, O_TRUNC: not modes on the fd at all.  These affect the meaning of open().  Heck, for openat2, NOCTTY should be this default.

O_EXCL: hopelessly overloaded.

O_APPEND, O_DIRECT, O_SYNC, O_DSYNC, O_LARGEFILE, O_NOATIME, O_PATH, O_NONBLOCK: genuine mode bits

O_CLOEXEC: special because it affects the fd, not the struct file.

Linus, you rejected resolveat() because you wanted a *nice* API that people would use and that might even be adopted by other OSes. Let’s please not make openat2() be a giant pile of crap in the name of consistency with open().  open(), frankly, is horrible.