[PATCH 0/6] kernel tinification: optionally compile out splice family of syscalls (splice, vmsplice, tee and sendfile)

REPO: https://github.com/smipi1/linux-tinification.git

BRANCH: tiny/config-syscall-splice

BACKGROUND: This patch-set forms part of the Linux Kernel Tinification effort (
  https://tiny.wiki.kernel.org/).

GOAL: Support compiling out the splice family of syscalls (splice, vmsplice,
  tee and sendfile) along with all supporting infrastructure if not needed.
  Many embedded systems will not need the splice-family syscalls. Omitting them
  saves space.

STRATEGY:
a. With the goal of eventually compiling out fs/splice.c, several functions
   that are only used in support of the the splice family of syscalls are moved
   into fs/splice.c from fs/read_write.c. The kernel_write function that is not
   used to support the splice syscalls is moved to fs/read_write.c:
     0001-fs-move-sendfile-syscall-into-fs-splice.patch
     0002-fs-moved-kernel_write-to-fs-read_write.patch

b. Introduce an EXPERT kernel configuration option; CONFIG_SYSCALL_SPLICE; to
   compile out the splice family of syscalls. This removes all userspace uses
   of the splice infrastructure.
     0003-fs-splice-support-compiling-out-splice-family-syscal.patch

c. Splice exports an operations struct, nosteal_pipe_buf_ops. Eliminate the 
   use of this struct when CONFIG_SYSCALL_SPLICE is undefined, so that splice
   can later be compiled out:
     0004-fs-fuse-support-compiling-out-splice.patch
     0005-net-core-support-compiling-out-splice.patch

e. Compile out fs/splice.c. Functions exported by fs/splice are mocked out with
   failing static inlines. This is done so as to all but eliminate the
   maintenance burden on file-system drivers.
     0006-fs-splice-full-support-for-compiling-out-splice.patch

RESULTS: A tinyconfig bloat-o-meter score for the entire patch-set:

add/remove: 0/41 grow/shrink: 5/7 up/down: 23/-8422 (-8399)
function                                     old     new   delta
sys_pwritev                                  115     122      +7
sys_preadv                                   115     122      +7
fdput_pos                                     29      36      +7
sys_pwrite64                                 115     116      +1
sys_pread64                                  115     116      +1
pipe_to_null                                   4       -      -4
generic_pipe_buf_nosteal                       6       -      -6
spd_release_page                              10       -     -10
fdput                                         11       -     -11
PageUptodate                                  22      11     -11
lock_page                                     36      24     -12
signal_pending                                39      26     -13
fdget                                         56      42     -14
page_cache_pipe_buf_release                   16       -     -16
user_page_pipe_buf_ops                        20       -     -20
splice_write_null                             24       4     -20
page_cache_pipe_buf_ops                       20       -     -20
nosteal_pipe_buf_ops                          20       -     -20
default_pipe_buf_ops                          20       -     -20
generic_splice_sendpage                       24       -     -24
user_page_pipe_buf_steal                      25       -     -25
splice_shrink_spd                             27       -     -27
pipe_to_user                                  43       -     -43
direct_splice_actor                           47       -     -47
default_file_splice_write                     49       -     -49
wakeup_pipe_writers                           54       -     -54
wakeup_pipe_readers                           54       -     -54
write_pipe_buf                                71       -     -71
page_cache_pipe_buf_confirm                   80       -     -80
splice_grow_spd                               87       -     -87
do_splice_to                                  87       -     -87
ipipe_prep.part                               92       -     -92
splice_from_pipe                              93       -     -93
splice_from_pipe_next                        107       -    -107
pipe_to_sendpage                             109       -    -109
page_cache_pipe_buf_steal                    114       -    -114
opipe_prep.part                              119       -    -119
sys_sendfile                                 122       -    -122
generic_file_splice_read                     131       8    -123
sys_sendfile64                               126       -    -126
sys_vmsplice                                 137       -    -137
do_splice_direct                             148       -    -148
vmsplice_to_user                             205       -    -205
__splice_from_pipe                           246       -    -246
splice_direct_to_actor                       348       -    -348
splice_to_pipe                               371       -    -371
do_sendfile                                  492       -    -492
sys_tee                                      497       -    -497
vmsplice_to_pipe                             558       -    -558
default_file_splice_read                     688       -    -688
iter_file_splice_write                       702       4    -698
sys_splice                                  1075       -   -1075
__generic_file_splice_read                  1109       -   -1109

Pieter Smith (6):
  fs: move sendfile syscall into fs/splice
  fs: moved kernel_write to fs/read_write
  fs/splice: support compiling out splice-family syscalls
  fs/fuse: support compiling out splice
  net/core: support compiling out splice
  fs/splice: full support for compiling out splice

 fs/Makefile            |   3 +-
 fs/fuse/dev.c          |   4 +-
 fs/read_write.c        | 181 +++------------------------------------------
 fs/splice.c            | 194 +++++++++++++++++++++++++++++++++++++++++++++----
 include/linux/fs.h     |  32 ++++++++
 include/linux/skbuff.h |   9 +++
 include/linux/splice.h |  42 +++++++++++
 init/Kconfig           |  10 +++
 kernel/sys_ni.c        |   8 ++
 net/core/skbuff.c      |   9 ++-
 10 files changed, 300 insertions(+), 192 deletions(-)

-- 
2.1.0

kernel_write shares infrastructure with the read_write translation unit but not
with the splice translation unit. Grouping kernel_write with the read_write
translation unit is more logical. It also paves the way to compiling out the
splice group of syscalls for embedded systems that do not need them.

Signed-off-by: Pieter Smith <redacted>
---
 fs/read_write.c | 16 ++++++++++++++++
 fs/splice.c     | 16 ----------------
 2 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index d9451ba..f4c8d8b 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c

@@ -1191,3 +1191,19 @@ COMPAT_SYSCALL_DEFINE5(pwritev, compat_ulong_t, fd,
 }
 #endif
 
+ssize_t kernel_write(struct file *file, const char *buf, size_t count,
+			    loff_t pos)
+{
+	mm_segment_t old_fs;
+	ssize_t res;
+
+	old_fs = get_fs();
+	set_fs(get_ds());
+	/* The cast to a user pointer is valid due to the set_fs() */
+	res = vfs_write(file, (__force const char __user *)buf, count, &pos);
+	set_fs(old_fs);
+
+	return res;
+}
+EXPORT_SYMBOL(kernel_write);
+

diff --git a/fs/splice.c b/fs/splice.c
index c1a2861..44b201b 100644
--- a/fs/splice.c
+++ b/fs/splice.c

@@ -583,22 +583,6 @@ static ssize_t kernel_readv(struct file *file, const struct iovec *vec,
 	return res;
 }
 
-ssize_t kernel_write(struct file *file, const char *buf, size_t count,
-			    loff_t pos)
-{
-	mm_segment_t old_fs;
-	ssize_t res;
-
-	old_fs = get_fs();
-	set_fs(get_ds());
-	/* The cast to a user pointer is valid due to the set_fs() */
-	res = vfs_write(file, (__force const char __user *)buf, count, &pos);
-	set_fs(old_fs);
-
-	return res;
-}
-EXPORT_SYMBOL(kernel_write);
-
 ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
 				 struct pipe_inode_info *pipe, size_t len,
 				 unsigned int flags)

-- 
2.1.0

Many embedded systems will not need the splice-family syscalls (splice,
vmsplice, tee and sendfile). Omitting them saves space.  This adds a new EXPERT
config option CONFIG_SYSCALL_SPLICE (default y) to support compiling them out.

The goal is to completely compile out fs/splice. To achieve this, the remaining
patch-set will deal with fs/splice exports. As far as possible, the impact on
other device drivers will be minimized so as to reduce the maintenance burden
of CONFIG_SYSCALL_SPLICE.

The use of exported functions will be solved by transparently mocking them out
with static inlines. Use of the exported pipe_buf_operations struct however
require direct modification in fs/fuse and net/core. The next two patches will
deal with this.

Once all exports are solved, fs/splice can be compiled out.

The bloat benefit of this patch given a tinyconfig is:

add/remove: 0/16 grow/shrink: 2/5 up/down: 114/-3693 (-3579)
function                                     old     new   delta
splice_direct_to_actor                       348     416     +68
splice_to_pipe                               371     417     +46
splice_from_pipe_next                        107     106      -1
fdput                                         11       -     -11
signal_pending                                39      26     -13
fdget                                         56      42     -14
user_page_pipe_buf_ops                        20       -     -20
user_page_pipe_buf_steal                      25       -     -25
file_end_write                                58      29     -29
file_start_write                              68      34     -34
pipe_to_user                                  43       -     -43
wakeup_pipe_readers                           54       -     -54
do_splice_to                                  87       -     -87
ipipe_prep.part                               92       -     -92
opipe_prep.part                              119       -    -119
sys_sendfile                                 122       -    -122
sys_sendfile64                               126       -    -126
sys_vmsplice                                 137       -    -137
vmsplice_to_user                             205       -    -205
sys_tee                                      491       -    -491
do_sendfile                                  492       -    -492
vmsplice_to_pipe                             558       -    -558
sys_splice                                  1020       -   -1020

Signed-off-by: Pieter Smith <redacted>
---
 fs/splice.c     |  2 ++
 init/Kconfig    | 10 ++++++++++
 kernel/sys_ni.c |  8 ++++++++
 3 files changed, 20 insertions(+)

diff --git a/fs/splice.c b/fs/splice.c
index 44b201b..7c4c695 100644
--- a/fs/splice.c
+++ b/fs/splice.c

@@ -1316,6 +1316,7 @@ long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
 	return ret;
 }
 
+#ifdef CONFIG_SYSCALL_SPLICE
 static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
 			       struct pipe_inode_info *opipe,
 			       size_t len, unsigned int flags);

@@ -2200,4 +2201,5 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int, in_fd,
 	return do_sendfile(out_fd, in_fd, NULL, count, 0);
 }
 #endif
+#endif
 

diff --git a/init/Kconfig b/init/Kconfig
index d811d5f..dec9819 100644
--- a/init/Kconfig
+++ b/init/Kconfig

@@ -1571,6 +1571,16 @@ config NTP
 	  system clock to an NTP server, you can disable this option to save
 	  space.
 
+config SYSCALL_SPLICE
+	bool "Enable splice/vmsplice/tee/sendfile syscalls" if EXPERT
+	default y
+	help
+	  This option enables the splice, vmsplice, tee and sendfile syscalls. These
+	  are used by applications to: move data between buffers and arbitrary file
+	  descriptors; "copy" data between buffers; or copy data from userspace into
+	  buffers. If building an embedded system where no applications use these
+	  syscalls, you can disable this option to save space.
+
 config PCI_QUIRKS
 	default y
 	bool "Enable PCI quirk workarounds" if EXPERT

diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index d2f5b00..25d5551 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c

@@ -170,6 +170,14 @@ cond_syscall(sys_fstat);
 cond_syscall(sys_stat);
 cond_syscall(sys_uname);
 cond_syscall(sys_olduname);
+cond_syscall(sys_vmsplice);
+cond_syscall(sys_splice);
+cond_syscall(sys_tee);
+cond_syscall(sys_sendfile);
+cond_syscall(sys_sendfile64);
+cond_syscall(compat_sys_vmsplice);
+cond_syscall(compat_sys_sendfile);
+cond_syscall(compat_sys_sendfile64);
 
 /* arch-specific weak syscall entries */
 cond_syscall(sys_pciconfig_read);

-- 
2.1.0

To implement splice support, fs/fuse makes use of nosteal_pipe_buf_ops. This
struct is exported by fs/splice. The goal of the larger patch set is to
completely compile out fs/splice, so uses of the exported struct need to be
compiled out along with fs/splice.

This patch therefore compiles out splice support in fs/fuse when
CONFIG_SYSCALL_SPLICE is undefined.

Signed-off-by: Pieter Smith <redacted>
---
 fs/fuse/dev.c      | 4 ++--
 include/linux/fs.h | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index ca88731..f8f92a4 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c

@@ -1291,7 +1291,7 @@ static ssize_t fuse_dev_read(struct kiocb *iocb, const struct iovec *iov,
 	return fuse_dev_do_read(fc, file, &cs, iov_length(iov, nr_segs));
 }
 
-static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
+static ssize_t __maybe_unused fuse_dev_splice_read(struct file *in, loff_t *ppos,
 				    struct pipe_inode_info *pipe,
 				    size_t len, unsigned int flags)
 {

@@ -2144,7 +2144,7 @@ const struct file_operations fuse_dev_operations = {
 	.llseek		= no_llseek,
 	.read		= do_sync_read,
 	.aio_read	= fuse_dev_read,
-	.splice_read	= fuse_dev_splice_read,
+	.splice_read	= __splice_p(fuse_dev_splice_read),
 	.write		= do_sync_write,
 	.aio_write	= fuse_dev_write,
 	.splice_write	= fuse_dev_splice_write,

diff --git a/include/linux/fs.h b/include/linux/fs.h
index a957d43..04c0975 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -2443,6 +2443,12 @@ extern int blkdev_fsync(struct file *filp, loff_t start, loff_t end,
 			int datasync);
 extern void block_sync_page(struct page *page);
 
+#ifdef CONFIG_SYSCALL_SPLICE
+#define __splice_p(x) x
+#else
+#define __splice_p(x) NULL
+#endif
+
 /* fs/splice.c */
 extern ssize_t generic_file_splice_read(struct file *, loff_t *,
 		struct pipe_inode_info *, size_t, unsigned int);

-- 
2.1.0