The decoder is flawed in the following ways:
- The decoder sometimes fails silently, i.e. it announces success but
returns a word that is not a codeword.
- The return value of the decoder is incoherent with respect to how
fixed erasures are counted. If the word to be decoded is a codeword,
then the decoder always returns zero even if some erasures are given.
On the other hand, if the word to be decoded contains errors, then the
number of erasures is always included in the count of corrected
symbols. So the decoder handles erasures without symbol corruption
inconsistently. This inconsistency probably doesn't affect anyone
using the decoder, but it is inconsistent with the documentation.
- The error positions returned in eras_pos include all erasures, but the
corrections are only set in the correction buffer if there actually is
a symbol error. So if there are erasures without symbol corruption,
then the correction buffer will contain errors (unless initialized to
zero before calling the decoder) or some values will be unset (if the
correction buffer is uninitialized).
- When correcting data in-place the decoder does not correct errors in
the parity. On the other hand, when returning the errors in correction
buffers, errors in the parity are included.
The respective fixed are:
- The syndrome of a codeword is always zero, and the syndrome is linear,
.i.e, S(x+e) = S(x) + S(e). So compute the syndrome for the error and
check whether it equals the syndrome of the received word. If it does,
then we have decoded to a valid codeword, otherwise we know that we
have an uncorrectable error. Fortunately, some unrecoverable error
conditions can be detected earlier in the decoding, which saves some
processing power.
- Simply count and return the number of symbols actually corrected.
- Make sure to only return positions where symbols were corrected.
- Also fix errors in parity when correcting in-place. Another option
would be to completely disregard errors in the parity, but then the
interface makes it impossible to write tests that test for silent
failures.
Other changes:
- Only fill the correction buffer and error position buffer if both of
them are provided. Otherwise correct in place. Previously the error
position buffer was always populated with the positions of the
corrected errors, irrespective of whether a correction buffer was
supplied or not. The rationale for this change is that there seems to
be two use cases for the decoder; correct in-place or use the
correction buffers. The caller does not need the positions of the
corrected errors when in-place correction is used. If in-place
correction is not used, then both the correction buffer and error
position buffer need to be populated.
Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-8-ferdinand.blomqvist@gmail.com
Nothing useful was done after the finish label when count is negative so
return directly instead of jumping to finish.
Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-5-ferdinand.blomqvist@gmail.com
The length of the data load must be at least one. Or in other words,
there must be room for at least 1 data and nroots parity symbols after
shortening the RS code.
Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-4-ferdinand.blomqvist@gmail.com
The decoding of shortenend codes is broken. It only works as expected if
there are no erasures.
When decoding with erasures, Lambda (the error and erasure locator
polynomial) is initialized from the given erasure positions. The pad
parameter is not accounted for by the initialisation code, and hence
Lambda is initialized from incorrect erasure positions.
The fix is to adjust the erasure positions by the supplied pad.
Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-3-ferdinand.blomqvist@gmail.com
To get rid of the variable length arrays on stack in the RS decoder it's
necessary to allocate the decoder buffers per control structure instance.
All usage sites have been checked for potential parallel decoder usage and
fixed where necessary. Kees confirmed that the pstore decoding is strictly
single threaded so there should be no surprises.
Allocate them in the rs control structure sized depending on the number of
roots for the chosen codec and adapt the decoder code to make use of them.
Document the fact that decode operations based on a particular rs control
instance cannot run in parallel and the caller has to ensure that as it's
not possible to provide a proper locking construct which fits all use
cases.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Andrew Morton <akpm@linuxfoundation.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alasdair Kergon <agk@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
The decoder library uses variable length arrays on stack. To get rid of
them it would be simple to allocate fixed length arrays on stack, but those
might become rather large. The other solution is to allocate the buffers in
the rs control structure, but this cannot be done as long as the structure
can be shared by several users. Sharing is desired because the RS polynom
tables are large and initialization is time consuming.
To solve this split the codec information out of the control structure and
have a pointer to a shared codec in it. Instantiate the control structure
for each user, create a new codec if no shareable is avaiable yet. Adjust
all affected usage sites to the new scheme.
This allows to add per instance decoder buffers to the control structure
later on.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Andrew Morton <akpm@linuxfoundation.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alasdair Kergon <agk@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
The Reed-Solomon library is based on code from Phil Karn who granted
permission to import it into the kernel under the GPL V2.
See commit 15b5423757a7 ("Shared Reed-Solomon ECC library") in the history
git tree at: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
...
The encoder/decoder code is lifted from the GPL'd userspace RS-library
written by Phil Karn. I modified/wrapped it to provide the different
functions which we need in the MTD/NAND code.
...
Signed-Off-By: Thomas Gleixner <tglx@linutronix.de>
Signed-Off-By: David Woodhouse <dwmw2@infradead.org>
"No objections at all. Just keep the authorship notices." -- Phil Karn
Add the proper SPDX identifiers according to
Documentation/process/license-rules.rst.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Andrew Morton <akpm@linuxfoundation.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alasdair Kergon <agk@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Magic numerical values are just bad style. Particularly so when
undocumented.
Signed-off-by: Jörn Engel <joern@logfs.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.
Let it rip!
Ferdinand Blomqvist