Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

Greg Kroah-Hartman
2020-03-03T14:10:35Z

On Tue, Mar 03, 2020 at 02:43:16PM +0100, Greg Kroah-Hartman wrote:
> On Tue, Mar 03, 2020 at 02:34:42PM +0100, Miklos Szeredi wrote:
> > On Tue, Mar 3, 2020 at 2:14 PM Greg Kroah-Hartman
> > <gregkh@linuxfoundation.org> wrote:
> >
> > > > Unlimited beers for a 21-line kernel patch? Sign me up!
> > > >
> > > > Totally untested, barely compiled patch below.
> > >
> > > Ok, that didn't even build, let me try this for real now...
> >
> > Some comments on the interface:
>
> Ok, hey, let's do this proper :)

Alright, how about this patch.

Actually tested with some simple sysfs files.

If people don't strongly object, I'll add "real" tests to it, hook it up
to all arches, write a manpage, and all the fun fluff a new syscall
deserves and submit it "for real".

It feels like I'm doing something wrong in that the actuall syscall
logic is just so small. Maybe I'll benchmark this thing to see if it
makes any real difference...

thanks,

greg k-h

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: [PATCH] readfile: implement readfile syscall

It's a tiny syscall, meant to allow a user to do a single "open this
file, read into this buffer, and close the file" all in a single shot.

Should be good for reading "tiny" files like sysfs, procfs, and other
"small" files.

There is no restarting the syscall, am trying to keep it simple. At
least for now.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
fs/open.c | 21 +++++++++++++++++++++
include/linux/syscalls.h | 2 ++
include/uapi/asm-generic/unistd.h | 4 +++-
5 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index c17cb77eb150..a79cd025e72b 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -442,3 +442,4 @@
435 i386 clone3 sys_clone3 __ia32_sys_clone3
437 i386 openat2 sys_openat2 __ia32_sys_openat2
438 i386 pidfd_getfd sys_pidfd_getfd __ia32_sys_pidfd_getfd
+439 i386 readfile sys_readfile __ia32_sys_readfile
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 44d510bc9b78..4f518f4e0e30 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 __x64_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..109bad47d542 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1340,3 +1340,24 @@ 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;
+
+ /* Mask off all O_ flags as we only want to read from the file */
+ flags &= ~(VALID_OPEN_FLAGS);
+ flags |= O_RDONLY | O_LARGEFILE;
+
+ fd = do_sys_open(dfd, filename, flags, 0000);
+ if (fd <= 0)
+ return fd;
+
+ retval = ksys_read(fd, buffer, bufsize);
+
+ __close_fd(current->files, fd);
+
+ return retval;
+}
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 1815065d52f3..3a636a913437 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1003,6 +1003,8 @@ asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
siginfo_t __user *info,
unsigned int flags);
asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
+asmlinkage long sys_readfile(int dfd, const char __user *filename,
+ char __user *buffer, size_t bufsize, int flags);

/*
* Architecture-specific system calls
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 3a3201e4618e..31f84500915d 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -855,9 +855,11 @@ __SYSCALL(__NR_clone3, sys_clone3)
__SYSCALL(__NR_openat2, sys_openat2)
#define __NR_pidfd_getfd 438
__SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
+#define __NR_readfile 439
+__SYSCALL(__NR_readfile, sys_readfile)

#undef __NR_syscalls
-#define __NR_syscalls 439
+#define __NR_syscalls 440

/*
* 32 bit systems traditionally used different
--
2.25.1