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Gianluca Borello 1612a981b7 bpf, x64: fix JIT emission for dead code 
Commit 2a5418a13f ("bpf: improve dead code sanitizing") replaced dead
code with a series of ja-1 instructions, for safety. That made JIT
compilation much more complex for some BPF programs. One instance of such
programs is, for example:

bool flag = false
...
/* A bunch of other code */
...
if (flag)
        do_something()

In some cases llvm is not able to remove at compile time the code for
do_something(), so the generated BPF program ends up with a large amount
of dead instructions. In one specific real life example, there are two
series of ~500 and ~1000 dead instructions in the program. When the
verifier replaces them with a series of ja-1 instructions, it causes an
interesting behavior at JIT time.

During the first pass, since all the instructions are estimated at 64
bytes, the ja-1 instructions end up being translated as 5 bytes JMP
instructions (0xE9), since the jump offsets become increasingly large (>
127) as each instruction gets discovered to be 5 bytes instead of the
estimated 64.

Starting from the second pass, the first N instructions of the ja-1
sequence get translated into 2 bytes JMPs (0xEB) because the jump offsets
become <= 127 this time. In particular, N is defined as roughly 127 / (5
- 2) ~= 42. So, each further pass will make the subsequent N JMP
instructions shrink from 5 to 2 bytes, making the image shrink every time.
This means that in order to have the entire program converge, there need
to be, in the real example above, at least ~1000 / 42 ~= 24 passes just
for translating the dead code. If we add this number to the passes needed
to translate the other non dead code, it brings such program to 40+
passes, and JIT doesn't complete. Ultimately the userspace loader fails
because such BPF program was supposed to be part of a prog array owner
being JITed.

While it is certainly possible to try to refactor such programs to help
the compiler remove dead code, the behavior is not really intuitive and it
puts further burden on the BPF developer who is not expecting such
behavior. To make things worse, such programs are working just fine in all
the kernel releases prior to the ja-1 fix.

A possible approach to mitigate this behavior consists into noticing that
for ja-1 instructions we don't really need to rely on the estimated size
of the previous and current instructions, we know that a -1 BPF jump
offset can be safely translated into a 0xEB instruction with a jump offset
of -2.

Such fix brings the BPF program in the previous example to complete again
in ~9 passes.

Fixes: 2a5418a13f ("bpf: improve dead code sanitizing")
Signed-off-by: Gianluca Borello <g.borello@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
 		2018-04-25 16:29:32 +02:00
Documentation bpf: Document sockmap '-target bpf' requirement for PROG_TYPE_SK_MSG 2018-04-23 23:42:21 +02:00
LICENSES LICENSES: Add MPL-1.1 license 2018-01-06 10:59:44 -07:00
arch bpf, x64: fix JIT emission for dead code 2018-04-25 16:29:32 +02:00
block blk-mq: Revert "blk-mq: reimplement blk_mq_hw_queue_mapped" 2018-04-11 07:59:15 -06:00
certs certs/blacklist_nohashes.c: fix const confusion in certs blacklist 2018-02-21 15:35:43 -08:00
crypto Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
drivers bonding: do not set slave_dev npinfo before slave_enable_netpoll in bond_enslave 2018-04-23 11:52:35 -04:00
firmware kbuild: remove all dummy assignments to obj- 2017-11-18 11:46:06 +09:00
fs vfs: Undo an overly zealous MS_RDONLY -> SB_RDONLY conversion 2018-04-20 09:59:33 -07:00
include bpf: sockmap, map_release does not hold refcnt for pinned maps 2018-04-24 00:49:45 +02:00
init Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-04-15 16:12:35 -07:00
ipc ipc/shm: fix use-after-free of shm file via remap_file_pages() 2018-04-13 17:10:27 -07:00
kernel bpf: sockmap, fix double page_put on ENOMEM error in redirect path 2018-04-24 00:49:45 +02:00
lib Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2018-04-20 09:34:39 -07:00
mm mm,vmscan: Allow preallocating memory for register_shrinker(). 2018-04-16 02:06:47 -04:00
net bpf: clear the ip_tunnel_info. 2018-04-25 09:51:54 +02:00
samples bpf: sockmap sample use clang flag, -target bpf 2018-04-23 23:42:21 +02:00
scripts Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
security + Features 2018-04-13 15:38:53 -07:00
sound amdgpu, omap and snd regression fix 2018-04-12 20:56:10 -07:00
tools Merge git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf 2018-04-22 21:15:59 -04:00
usr kbuild: rename built-in.o to built-in.a 2018-03-26 02:01:19 +09:00
virt KVM/ARM updates for v4.17 2018-03-28 16:09:09 +02:00
.clang-format clang-format: add configuration file 2018-04-11 10:28:35 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Add hch to .get_maintainer.ignore 2015-08-21 14:30:10 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
.mailmap Merge candidates for 4.17 merge window 2018-04-06 17:35:43 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS/CREDITS: Drop METAG ARCHITECTURE 2018-03-05 16:34:24 +00:00
Kbuild Kbuild updates for v4.15 2017-11-17 17:45:29 -08:00
Kconfig License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
MAINTAINERS Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2018-04-20 09:34:39 -07:00
Makefile Linux 4.17-rc1 2018-04-15 18:24:20 -07:00
README Docs: Added a pointer to the formatted docs to README 2018-03-21 09:02:53 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.