linux-kernel - Re: [PATCH v2 2/2] selftests/nolibc: add testcase for pipe

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Message-ID: <CEF9B5A6B5C43FBB+a5f21a530c1baf5229614c8186043e223b9d111f.camel@tinylab.org>
Date:   Tue, 01 Aug 2023 20:23:02 +0800
From:   Yuan Tan <tanyuan@...ylab.org>
To:     Thomas Weißschuh <thomas@...ch.de>
Cc:     w@....eu, falcon@...ylab.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, tanyuan@...ylab.org
Subject: Re: [PATCH v2 2/2] selftests/nolibc: add testcase for pipe

Hi Thomas,

On Tue, 2023-08-01 at 09:20 +0200, Thomas Weißschuh wrote:
> On 2023-08-01 14:51:40+0800, Yuan Tan wrote:
> > Hi Thomas,
> > 
> > On Mon, 2023-07-31 at 20:28 +0200, Thomas Weißschuh wrote:
> > > On 2023-08-01 02:01:36+0800, Yuan Tan wrote:
> > > > Hi Thomas,
> > > > On Mon, 2023-07-31 at 17:41 +0200, Thomas Weißschuh wrote:
> > > > > On 2023-07-31 20:35:28+0800, Yuan Tan wrote:
> > > > > > On Mon, 2023-07-31 at 08:10 +0200, Thomas Weißschuh wrote:
> > > > > > > On 2023-07-31 13:51:00+0800, Yuan Tan wrote:
> > > > > > > > Add a testcase of pipe that child process sends message
> > > > > > > > to
> > > > > > > > parent
> > > > > > > > process.
> > > > > > > 
> > > > > > > Thinking about it some more:
> > > > > > > 
> > > > > > > What's the advantage of going via a child process?
> > > > > > > The pipe should work the same within the same process.
> > > > > > > 
> > > > > > 
> > > > > > The pipe is commonly used for process communication, and I
> > > > > > think as
> > > > > > a
> > > > > > test case it is supposed to cover the most common
> > > > > > scenarios.
> > > > > 
> > > > > The testcase is supposed to cover the code of nolibc.
> > > > > It should be the *minimal* amount of code to be reasonable
> > > > > sure
> > > > > that
> > > > > the
> > > > > code in nolibc does the correct thing.
> > > > > If pipe() returns a value that behaves like a pipe I see no
> > > > > reason to
> > > > > doubt it will also survive fork().
> > > > > 
> > > > > Validating that would mean testing the kernel and not nolibc.
> > > > > For the kernel there are different testsuites.
> > > > > 
> > > > > Less code means less work for everyone involved, now and in
> > > > > the
> > > > > future.
> > > > > 
> > > > 
> > > > It's a good point and I never thought about this aspect.
> > > > 
> > > > I wonder whether the code below is enough?
> > > > 
> > > > static int test_pipe(void)
> > > > {
> > > >         int pipefd[2];
> > > > 
> > > >         if (pipe(pipefd) == -1)
> > > >                 return 1;
> > > > 
> > > >         close(pipefd[0]);
> > > >         close(pipefd[1]);
> > > > 
> > > >         return 0;
> > > > }
> > > 
> > > That is very barebones.
> > > 
> > > If accidentally a wrong syscall number was used and the used
> > > syscall
> > > would not take any arguments this test would still succeed.
> > > 
> > > Keeping the write-read-cycle from the previous revision would
> > > test
> > > that
> > > nicely. Essentially the same code as before but without the
> > > fork().
> > > 
> > > > 
> > > > And I forgot to add this line:
> > > > 
> > > >         CASE_TEST(pipe); EXPECT_SYSZR(1, test_pipe()); break;
> > > > 
> > > > I will add it in next patch.
> > > > 
> > > 
> > 
> > In the situation you described, that is indeed the case.
> > 
> > Would this be fine?
> > 
> > static int test_pipe(void)
> > {
> >         const char *const msg = "hello, nolibc";
> >         int pipefd[2];
> >         char buf[32];
> >         ssize_t len;
> > 
> >         if (pipe(pipefd) == -1)
> >                 return 1;
> > 
> >         write(pipefd[1], msg, strlen(msg));
> >         close(pipefd[1]);
> >         len = read(pipefd[0], buf, sizeof(buf));
> >         close(pipefd[0]);
> > 
> >         if (len != strlen(msg))
> >                 return 1;
> > 
> >         return !!memcmp(buf, msg, len);
> > }
> 
> Looks good!
> 
> The return value of write() could also be validated but given we
> validate the return value from read() it shouldn't make a difference.
> 
> (Also the manual manipulation of "buf" is gone that necessitated the
> check in v1 of the series)
> 

I am sorry that I didn't catch your last sentence.

Did you mean this piece of code does not need any further modifications
right?