If the data passed to sys$write() is backed by a not-yet-paged-in inode
mapping, we could end up in a situation where we get a page fault when
trying to copy data from userspace.
If that page fault handler tried reading from an inode that someone else
had locked while waiting for the disk cache lock, we'd deadlock.
This patch fixes the issue by copying the userspace data into a local
buffer before acquiring the disk cache lock. This is not ideal since it
incurs an extra copy, and I'm sure we can think of a better solution
eventually.
This was a frequent cause of startup deadlocks on x86_64 for me. :^)
We now use AK::Error and AK::ErrorOr<T> in both kernel and userspace!
This was a slightly tedious refactoring that took a long time, so it's
not unlikely that some bugs crept in.
Nevertheless, it does pass basic functionality testing, and it's just
real nice to finally see the same pattern in all contexts. :^)
Create the disk cache up front, so we can verify it succeeds.
Make the KBuffer allocation fail-able, so we can properly handle
failure when the user asks up to mount a Ext2 filesystem under
OOM conditions.
The file system lock is meant to protect the file system metadata
(super blocks, bitmaps, etc.) Not protect processes from reading
independent parts of the disk at once.
This patch introduces a new lock to protect the *block cache* instead,
which is the real thing that needs synchronization.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
This should allow creating intrusive lists that have smart pointers,
while remaining free (compared to the impl before this commit) when
holding raw pointers :^)
As a sidenote, this also adds a `RawPtr<T>` type, which is just
equivalent to `T*`.
Note that this does not actually use such functionality, but is only
expected to pave the way for #6369, to replace NonnullRefPtrVector<T>
with intrusive lists.
As it is with zero-cost things, this makes the interface a bit less nice
by requiring the type name of what an `IntrusiveListNode` holds (and
optionally its container, if not RawPtr), and also requiring the type of
the container (normally `RawPtr`) on the `IntrusiveList` instance.
This may seem like a no-op change, however it shrinks down the Kernel by a bit:
.text -432
.unmap_after_init -60
.data -480
.debug_info -673
.debug_aranges 8
.debug_ranges -232
.debug_line -558
.debug_str -308
.debug_frame -40
With '= default', the compiler can do more inlining, hence the savings.
I intentionally omitted some opportunities for '= default', because they
would increase the Kernel size.
Since these filesystems operate on an underlying file descriptor
and rely on its offset for correctness, let's use the FS lock to
serialize these operations.
This also means that FS subclasses can rely on block-level read/write
operations being atomic.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
..and allow implicit creation of KResult and KResultOr from ErrnoCode.
This means that kernel functions that return those types can finally
do "return EINVAL;" and it will just work.
There's a handful of functions that still deal with signed integers
that should be converted to return KResults.
This way, if something goes wrong, we get to keep the actual error.
Also, KResults are nodiscard, so we have to deal with that in Ext2FS
instead of just silently ignoring I/O errors(!)
BlockBasedFileSystem::read_block method should get a reference of
a UserOrKernelBuffer.
If we need to force caching a block, we will call other method to do so.
Problem:
- `(void)` simply casts the expression to void. This is understood to
indicate that it is ignored, but this is really a compiler trick to
get the compiler to not generate a warning.
Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.
Note:
- Functions taking a `(void)` argument list have also been changed to
`()` because this is not needed and shows up in the same grep
command.
This makes misses in the BlockBasedFS's LRU block cache faster by
storing the cache entries in one of two doubly-linked list.
Dirty and clean cache entries are kept in two separate lists, and
move between them when their state changes. This can probably be
improved upon further.
Instead of doing a linear scan of the entire cache when doing a lookup,
we now have a nice O(1) HashMap in front of the cache.
The cache miss case can still be improved, this patch really only helps
the cache hit case.
This dramatically improves cached filesystem I/O. :^)
Since the CPU already does almost all necessary validation steps
for us, we don't really need to attempt to do this. Doing it
ourselves doesn't really work very reliably, because we'd have to
account for other processors modifying virtual memory, and we'd
have to account for e.g. pages not being able to be allocated
due to insufficient resources.
So change the copy_to/from_user (and associated helper functions)
to use the new safe_memcpy, which will return whether it succeeded
or not. The only manual validation step needed (which the CPU
can't perform for us) is making sure the pointers provided by user
mode aren't pointing to kernel mappings.
To make it easier to read/write from/to either kernel or user mode
data add the UserOrKernelBuffer helper class, which will internally
either use copy_from/to_user or directly memcpy, or pass the data
through directly using a temporary buffer on the stack.
Last but not least we need to keep syscall params trivial as we
need to copy them from/to user mode using copy_from/to_user.
FileBackedFileSystem is one that's backed by (mounted from) a file, in other
words one that has a "source" of the mount; that doesn't mean it deals in
blocks. The hierarchy now becomes:
* FS
* ProcFS
* DevPtsFS
* TmpFS
* FileBackedFS
* (future) Plan9FS
* BlockBasedFS
* Ext2FS
