This amends commit 10dce8af ("fs: stream_open - opener for
stream-like files so that read and write can run simultaneously without
deadlock") in how position is passed into .read()/.write() handler for
stream-like files:

Rasmus noticed that we currently pass 0 as position and ignore any position
change if that is done by a file implementation. This papers over bugs if ppos
is used in files that declare themselves as being stream-like as such bugs will
go unnoticed. Even if a file implementation is correctly converted into using
stream_open, its read/write later could be changed to use ppos and even though
that won't be working correctly, that bug might go unnoticed without someone
doing wrong behaviour analysis. It is thus better to pass ppos=NULL into
read/write for stream-like files as that don't give any chance for ppos usage
bugs because it will oops if ppos is ever used inside .read() or .write().

Rasmus also made the point that FMODE_STREAM patch added 2 branches into
ksys_read/ksys_write patch which is too much unnecessary overhead and
could be reduces.

Let's rework the code to pass ppos=NULL and have less branches.

The first approach could be to revert file_pos_read and file_pos_write
into their pre-FMODE_STREAM version and just do

	--- a/fs/read_write.c
	+++ b/fs/read_write.c
	@@ -575,7 +575,7 @@ ssize_t ksys_read(unsigned int fd, char __user *buf,
	size_t count)

	        if (f.file) {
	                loff_t pos = file_pos_read(f.file);
	-               ret = vfs_read(f.file, buf, count, &pos);
	+               ret = vfs_read(f.file, buf, count, (file->f_mode & FMODE_STREAM) ? NULL : &pos);
	                if (ret >= 0)
	                        file_pos_write(f.file, pos);
	                fdput_pos(f);

this adds no branches at all as `(file->f_mode & FMODE_STREAM) ? NULL : &pos)`
translates to single cmov instruction on x86_64:

        if (f.file) {
    18e5:       48 89 c3                mov    %rax,%rbx
    18e8:       48 83 e3 fc             and    $0xfffffffffffffffc,%rbx
    18ec:       74 79                   je     1967 <ksys_read+0xa7>
    18ee:       48 89 c5                mov    %rax,%rbp
                loff_t pos = f.file->f_pos;
    18f1:       48 8b 43 68             mov    0x68(%rbx),%rax
                ret = vfs_read(f.file, buf, count, (f.file->f_mode & FMODE_STREAM) ? NULL : &pos);
    18f5:       48 89 e1                mov    %rsp,%rcx
    18f8:       f6 43 46 20             testb  $0x20,0x46(%rbx)
    18fc:       4c 89 e6                mov    %r12,%rsi
    18ff:       4c 89 ea                mov    %r13,%rdx
    1902:       48 89 df                mov    %rbx,%rdi
                loff_t pos = f.file->f_pos;
    1905:       48 89 04 24             mov    %rax,(%rsp)
                ret = vfs_read(f.file, buf, count, (f.file->f_mode & FMODE_STREAM) ? NULL : &pos);
    1909:       b8 00 00 00 00          mov    $0x0,%eax
    190e:       48 0f 45 c8             cmovne %rax,%rcx		<-- NOTE
    1912:       e8 00 00 00 00          callq  1917 <ksys_read+0x57>
                        1913: R_X86_64_PLT32    vfs_read-0x4

However this leaves on unconditional write into file->f_pos after
vfs_read call, and even though it should not be affecting semantic, it
will bug under race detector (e.g. KTSAN) and probably it might add some
inter-CPU overhead if read and write handlers are running on different
cores.

If we instead move `file->f_mode & FMODE_STREAM` checking into vfs
functions that actually dispatch IO and care to load/store file->f_fpos
only for non-stream files, even though it is 3 branches if looking at C
source, gcc generates only 1 more branch compared to how it was
pre-FMODE_STREAM. For example consider updated ksys_read:

	 ssize_t ksys_read(unsigned int fd, char __user *buf, size_t count)
	 {
	 	struct fd f = fdget_pos(fd);
	 	ssize_t ret = -EBADF;

	 	if (f.file) {
	-		loff_t pos = file_pos_read(f.file);
	-		ret = vfs_read(f.file, buf, count, &pos);
	-		if (ret >= 0)
	-			file_pos_write(f.file, pos);
	+		loff_t pos, *ppos = f.file->f_mode & FMODE_STREAM ? NULL : &pos;
	+		if (ppos)
	+			pos = f.file->f_pos;
	+		ret = vfs_read(f.file, buf, count, ppos);
	+		if (ret >= 0 && ppos)
	+			f.file->f_pos = pos;
	 		fdput_pos(f);
	 	}
	 	return ret;

The code that gcc generates here is essentially as if the source was:

	if (f.file->f_mode & FMODE_STREAM) {
		ret = vfs_read(f.file, buf, count, NULL);
	} else {
		loff_t pos = f.file->f_pos;
		ret = vfs_read(f.file, buf, count, ppos);
		if (ret >= 0)
			f.file->f_pos = pos;
	}
	fdput_pos(f);

i.e. it branches only once after checking file->f_mode and then has two
distinct codepaths each with its own vfs_read call:

        if (f.file) {
    18e5:       48 89 c5                mov    %rax,%rbp
    18e8:       48 83 e5 fc             and    $0xfffffffffffffffc,%rbp
    18ec:       0f 84 89 00 00 00       je     197b <ksys_read+0xbb>
    18f2:       48 89 c3                mov    %rax,%rbx
                loff_t pos, *ppos = (f.file->f_mode & FMODE_STREAM) ? NULL : &pos;
    18f5:       f6 45 46 20             testb  $0x20,0x46(%rbp)
    18f9:       74 3b                   je     1936 <ksys_read+0x76>	<-- branch pos/no-pos
                ret = vfs_read(f.file, buf, count, ppos);
    18fb:       4c 89 e6                mov    %r12,%rsi		<-- start of IO without pos
    18fe:       31 c9                   xor    %ecx,%ecx
    1900:       4c 89 ea                mov    %r13,%rdx
    1903:       48 89 ef                mov    %rbp,%rdi
    1906:       e8 00 00 00 00          callq  190b <ksys_read+0x4b>
                        1907: R_X86_64_PLT32    vfs_read-0x4		<-- vfs_read(..., ppos=NULL)
    190b:       49 89 c4                mov    %rax,%r12
        if (f.flags & FDPUT_POS_UNLOCK)
    190e:       f6 c3 02                test   $0x2,%bl
    1911:       75 51                   jne    1964 <ksys_read+0xa4>
        if (fd.flags & FDPUT_FPUT)
    1913:       83 e3 01                and    $0x1,%ebx
    1916:       75 59                   jne    1971 <ksys_read+0xb1>
}
    1918:       48 8b 4c 24 08          mov    0x8(%rsp),%rcx
    191d:       65 48 33 0c 25 28 00    xor    %gs:0x28,%rcx
    1924:       00 00
    1926:       4c 89 e0                mov    %r12,%rax
    1929:       75 59                   jne    1984 <ksys_read+0xc4>
    192b:       48 83 c4 10             add    $0x10,%rsp
    192f:       5b                      pop    %rbx
    1930:       5d                      pop    %rbp
    1931:       41 5c                   pop    %r12
    1933:       41 5d                   pop    %r13
    1935:       c3                      retq
                        pos = f.file->f_pos;
    1936:       48 8b 45 68             mov    0x68(%rbp),%rax		<-- start of IO with pos
                ret = vfs_read(f.file, buf, count, ppos);
    193a:       4c 89 e6                mov    %r12,%rsi
    193d:       48 89 e1                mov    %rsp,%rcx
    1940:       4c 89 ea                mov    %r13,%rdx
    1943:       48 89 ef                mov    %rbp,%rdi
                        pos = f.file->f_pos;
    1946:       48 89 04 24             mov    %rax,(%rsp)
                ret = vfs_read(f.file, buf, count, ppos);
    194a:       e8 00 00 00 00          callq  194f <ksys_read+0x8f>
                        194b: R_X86_64_PLT32    vfs_read-0x4		<-- vfs_read(..., ppos=&pos)
    194f:       49 89 c4                mov    %rax,%r12
                if (ret >= 0 && ppos)
    1952:       48 85 c0                test   %rax,%rax
    1955:       78 b7                   js     190e <ksys_read+0x4e>
                        f.file->f_pos = pos;
    1957:       48 8b 04 24             mov    (%rsp),%rax
    195b:       48 89 45 68             mov    %rax,0x68(%rbp)
        if (f.flags & FDPUT_POS_UNLOCK)
    ...

If adding one branch to handle FMODE_STREAM is acceptable, this approach should
be ok. Not duplicating vfs_read calls at C level is better as C-level code is
more clear without such duplication, and gcc cares to optimize the function
properly to have only 1 branch depending on file->f_mode.

If even 1 extra branch is unacceptable, we should indeed go with the first
option described in this patch but be prepared for race-detector bug reports
and probably some inter-CPU overhead.

Overal I would suggest to use simpler approach presented by hereby patch unless
1-extra jump could be shown to cause noticable slowdowns in practice.
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>