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

Greg Kroah-Hartman
2020-03-03T13:03:52Z

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;
+}