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Date:   Tue, 3 Mar 2020 14:03:47 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Karel Zak <kzak@redhat.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        David Howells <dhowells@redhat.com>,
        Ian Kent <raven@themaw.net>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver
 #17]
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On Tue, Mar 03, 2020 at 12:38:14PM +0100, Karel Zak wrote:
> On Tue, Mar 03, 2020 at 10:26:21AM +0100, Miklos Szeredi wrote:
> > No, I don't think this is going to be a performance issue at all, but
> > if anything we could introduce a syscall
> > 
> >   ssize_t readfile(int dfd, const char *path, char *buf, size_t
> > bufsize, int flags);
> 
> off-topic, but I'll buy you many many beers if you implement it ;-),
> because open + read + close is pretty common for /sys and /proc in
> many userspace tools; for example ps, top, lsblk, lsmem, lsns, udevd
> etc. is all about it.

Unlimited beers for a 21-line kernel patch?  Sign me up!

Totally untested, barely compiled patch below.

Actually, I like this idea (the syscall, not just the unlimited beers).
Maybe this could make a lot of sense, I'll write some actual tests for
it now that syscalls are getting "heavy" again due to CPU vendors
finally paying the price for their madness...

thanks,

greg k-h
-------------------


diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 44d510bc9b78..178cd45340e2 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -359,6 +359,7 @@
 435	common	clone3			__x64_sys_clone3/ptregs
 437	common	openat2			__x64_sys_openat2
 438	common	pidfd_getfd		__x64_sys_pidfd_getfd
+439	common	readfile		__x86_sys_readfile
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/fs/open.c b/fs/open.c
index 0788b3715731..1a830fada750 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1340,3 +1340,23 @@ int stream_open(struct inode *inode, struct file *filp)
 }
 
 EXPORT_SYMBOL(stream_open);
+
+SYSCALL_DEFINE5(readfile, int, dfd, const char __user *, filename,
+		char __user *, buffer, size_t, bufsize, int, flags)
+{
+	int retval;
+	int fd;
+
+	if (force_o_largefile())
+		flags |= O_LARGEFILE;
+
+	fd = do_sys_open(dfd, filename, flags, O_RDONLY);
+	if (fd <= 0)
+		return fd;
+
+	retval = ksys_read(fd, buffer, bufsize);
+
+	__close_fd(current->files, fd);
+
+	return retval;
+}