linux-kernel - Re: [PATCH 2/2] selftests/nolibc: add testcase for pipe.

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Message-ID: <c5de2d13-3752-4e1b-90d9-f58cca99c702@t-8ch.de>
Date:   Sun, 30 Jul 2023 10:07:24 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Willy Tarreau <w@....eu>
Cc:     Yuan Tan <tanyuan@...ylab.org>, falcon@...ylab.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/2] selftests/nolibc: add testcase for pipe.

On 2023-07-30 09:12:27+0200, Willy Tarreau wrote:
> On Sun, Jul 30, 2023 at 08:55:47AM +0200, Thomas Weißschuh wrote:
> > On 2023-07-30 05:33:43+0200, Willy Tarreau wrote:
> > > On Sun, Jul 30, 2023 at 12:17:24AM +0200, Thomas Weißschuh wrote:
> > > > > +	case 0:
> > > > > +		close(pipefd[0]);
> > > > > +		write(pipefd[1], msg, strlen(msg));
> > > > 
> > > > Isn't this missing to write trailing the 0 byte?
> > > 
> > > It depends if the other side expects to get the trailing 0.
> > > In general it's better to avoid sending it since it's only
> > > used for internal representation, and the other side must
> > > be prepared to receive anything anyway.
> > > 
> > > > Also check the return value.
> > > 
> > > Indeed!
> > > 
> > > > > +		close(pipefd[1]);
> > > > 
> > > > Do we need to close the pipefds? The process is exiting anyways.
> > > 
> > > It's better to, because we could imagine looping over the tests for
> > > example. Thus each test shoulld have as little impact as possible
> > > on other tests.
> > 
> > I meant the newly forked child exiting, not nolibc-test in general.
> > The exit is just below, so the fds in the child are close here anyways.
> 
> Ah OK, but still it remains cleaner with it IMHO (i.e. better rely on
> explicit things in tests, that's less doubts when they fail).

Accepted :-)

> > > > > +	default:
> > > > > +		close(pipefd[1]);
> > > > > +		read(pipefd[0], buf, 32);
> > > > 
> > > > Use sizeof(buf). Check return value == strlen(msg).
> > > > 
> > > > > +		close(pipefd[0]);
> > > > > +		wait(NULL);
> > > > 
> > > > waitpid(pid, NULL, 0);
> > > > 
> > > > > +
> > > > > +		if (strcmp(buf, msg))
> > > > > +			return 1;
> > > > > +		return 0;
> > > > 
> > > > return !!strcmp(buf, msg);
> > > 
> > > In fact before that we need to terminate the output buffer. If for any
> > > reason the transfer fails (e.g. the syscall fails or transfers data at
> > > another location or of another length, we could end up comparing past
> > > the end of the buffer. Thus I suggest adding this immediately after the
> > > read():
> > > 
> > > 		buf[sizeof(buf) - 1] = 0;
> > 
> > This would still access uninitialized memory and lead to UB in strcmp as
> > not all bytes in buf were written to by read().
> > 
> > If we want to be really sure we should use memcmp() instead of strcmp().
> > For memcmp() I would prefer to transfer and check without the '\0', so
> > my review comments from before need to be adapted a bit.
> 
> In fact you make a good point regarding the fact that the test doesn't
> use read()'s return value. This problem totally goes away if the return
> value is used, e.g.:
> 
>       len = read(pipefd[0], buf, sizeof(buf));
>       close(pipefd[0]);
>       waitpid(pid, NULL, 0);
>       return len < 0 || len > sizeof(buf) || len > strlen(msg) || memcmp(buf, msg, len) != 0;

Wouldn't this happily accept len == 0?

Why not just:

if (len != strlen(msg))
  return 1;
return !!memcmp(buf, msg, len);

Also so far we have assumed that one call one call to read() is enough.
But looking at pipe(7) this is not guaranteed by the spec.
If we want to be really sure, a loop around read() seems to be necessary.