- Enable resize on mmap() error
When a process mmaps a ring buffer, its size is locked and resizing is
disabled. But if the user passes in a wrong parameter, the mmap() can fail
after the resize was disabled and the mmap() exits with error without
reenabling the ring buffer resize. This prevents the ring buffer from ever
being resized after that. Reenable resizing of the ring buffer on mmap()
error.
- Have resizing return proper error and not always -ENOMEM
If the ring buffer is mmapped by one task and another task tries to resize
the buffer it will error with -ENOMEM. This is confusing to the user as
there may be plenty of memory available. Have it return the error that
actually happens (in this case -EBUSY) where the user can understand why
the resize failed.
- Test the sub-buffer array to validate persistent memory buffer
On boot up, the initialization of the persistent memory buffer will do a
validation check to see if the content of the data is valid, and if so, it
will use the memory as is, otherwise it re-initializes it. There's meta
data in this persistent memory that keeps track of which sub-buffer is the
reader page and an array that states the order of the sub-buffers. The
values in this array are indexes into the sub-buffers. The validator
checks to make sure that all the entries in the array are within the
sub-buffer list index, but it does not check for duplications.
While working on this code, the array got corrupted and had duplicates,
where not all the sub-buffers were accounted for. This passed the
validator as all entries were valid, but the link list was incorrect and
could have caused a crash. The corruption only produced incorrect data,
but it could have been more severe. To fix this, create a bitmask that
covers all the sub-buffer indexes and set it to all zeros. While iterating
the array checking the values of the array content, have it set a bit
corresponding to the index in the array. If the bit was already set, then
it is a duplicate and mark the buffer as invalid and reset it.
- Prevent mmap()ing persistent ring buffer
The persistent ring buffer uses vmap() to map the persistent memory.
Currently, the mmap() logic only uses virt_to_page() to get the page
from the ring buffer memory and use that to map to user space. This works
because a normal ring buffer uses alloc_page() to allocate its memory.
But because the persistent ring buffer use vmap() it causes a kernel
crash. Fixing this to work with vmap() is not hard, but since mmap() on
persistent memory buffers never worked, just have the mmap() return
-ENODEV (what was returned before mmap() for persistent memory ring
buffers, as they never supported mmap. Normal buffers will still allow
mmap(). Implementing mmap() for persistent memory ring buffers can wait
till the next merge window.
- Fix polling on persistent ring buffers
There's a "buffer_percent" option (default set to 50), that is used to
have reads of the ring buffer binary data block until the buffer fills to
that percentage. The field "pages_touched" is incremented every time a
new sub-buffer has content added to it. This field is used in the
calculations to determine the amount of content is in the buffer and if it
exceeds the "buffer_percent" then it will wake the task polling on the
buffer.
As persistent ring buffers can be created by the content from a previous
boot, the "pages_touched" field was not updated. This means that if a task
were to poll on the persistent buffer, it would block even if the buffer
was completely full. It would block even if the "buffer_percent" was zero,
because with "pages_touched" as zero, it would be calculated as the buffer
having no content. Update pages_touched when initializing the persistent
ring buffer from a previous boot.
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Merge tag 'trace-ring-buffer-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
Pull trace ring buffer fixes from Steven Rostedt:
- Enable resize on mmap() error
When a process mmaps a ring buffer, its size is locked and resizing
is disabled. But if the user passes in a wrong parameter, the mmap()
can fail after the resize was disabled and the mmap() exits with
error without reenabling the ring buffer resize. This prevents the
ring buffer from ever being resized after that. Reenable resizing of
the ring buffer on mmap() error.
- Have resizing return proper error and not always -ENOMEM
If the ring buffer is mmapped by one task and another task tries to
resize the buffer it will error with -ENOMEM. This is confusing to
the user as there may be plenty of memory available. Have it return
the error that actually happens (in this case -EBUSY) where the user
can understand why the resize failed.
- Test the sub-buffer array to validate persistent memory buffer
On boot up, the initialization of the persistent memory buffer will
do a validation check to see if the content of the data is valid, and
if so, it will use the memory as is, otherwise it re-initializes it.
There's meta data in this persistent memory that keeps track of which
sub-buffer is the reader page and an array that states the order of
the sub-buffers. The values in this array are indexes into the
sub-buffers. The validator checks to make sure that all the entries
in the array are within the sub-buffer list index, but it does not
check for duplications.
While working on this code, the array got corrupted and had
duplicates, where not all the sub-buffers were accounted for. This
passed the validator as all entries were valid, but the link list was
incorrect and could have caused a crash. The corruption only produced
incorrect data, but it could have been more severe. To fix this,
create a bitmask that covers all the sub-buffer indexes and set it to
all zeros. While iterating the array checking the values of the array
content, have it set a bit corresponding to the index in the array.
If the bit was already set, then it is a duplicate and mark the
buffer as invalid and reset it.
- Prevent mmap()ing persistent ring buffer
The persistent ring buffer uses vmap() to map the persistent memory.
Currently, the mmap() logic only uses virt_to_page() to get the page
from the ring buffer memory and use that to map to user space. This
works because a normal ring buffer uses alloc_page() to allocate its
memory. But because the persistent ring buffer use vmap() it causes a
kernel crash.
Fixing this to work with vmap() is not hard, but since mmap() on
persistent memory buffers never worked, just have the mmap() return
-ENODEV (what was returned before mmap() for persistent memory ring
buffers, as they never supported mmap. Normal buffers will still
allow mmap(). Implementing mmap() for persistent memory ring buffers
can wait till the next merge window.
- Fix polling on persistent ring buffers
There's a "buffer_percent" option (default set to 50), that is used
to have reads of the ring buffer binary data block until the buffer
fills to that percentage. The field "pages_touched" is incremented
every time a new sub-buffer has content added to it. This field is
used in the calculations to determine the amount of content is in the
buffer and if it exceeds the "buffer_percent" then it will wake the
task polling on the buffer.
As persistent ring buffers can be created by the content from a
previous boot, the "pages_touched" field was not updated. This means
that if a task were to poll on the persistent buffer, it would block
even if the buffer was completely full. It would block even if the
"buffer_percent" was zero, because with "pages_touched" as zero, it
would be calculated as the buffer having no content. Update
pages_touched when initializing the persistent ring buffer from a
previous boot.
* tag 'trace-ring-buffer-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
ring-buffer: Update pages_touched to reflect persistent buffer content
tracing: Do not allow mmap() of persistent ring buffer
ring-buffer: Validate the persistent meta data subbuf array
tracing: Have the error of __tracing_resize_ring_buffer() passed to user
ring-buffer: Unlock resize on mmap error
5784d8c93e