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Our current reference leak checker uses finalizers to verify whether an object
has reached zero references before it is garbage collected. There are multiple
problems with this mechanism, so a rewrite is in order.
With finalizers, there is no way to guarantee that a finalizer will run before
the program exits. When an unreachable object with a finalizer is garbage
collected, its finalizer will be added to a queue and run asynchronously. The
best we can do is run garbage collection upon sandbox exit to make sure that
all finalizers are enqueued.
Furthermore, if there is a chain of finalized objects, e.g. A points to B
points to C, garbage collection needs to run multiple times before all of the
finalizers are enqueued. The first GC run will register the finalizer for A but
not free it. It takes another GC run to free A, at which point B's finalizer
can be registered. As a result, we need to run GC as many times as the length
of the longest such chain to have a somewhat reliable leak checker.
Finally, a cyclical chain of structs pointing to one another will never be
garbage collected if a finalizer is set. This is a well-known issue with Go
finalizers (https://github.com/golang/go/issues/7358). Using leak checking on
filesystem objects that produce cycles will not work and even result in memory
leaks.
The new leak checker stores reference counted objects in a global map when
leak check is enabled and removes them once they are destroyed. At sandbox
exit, any remaining objects in the map are considered as leaked. This provides
a deterministic way of detecting leaks without relying on the complexities of
finalizers and garbage collection.
This approach has several benefits over the former, including:
- Always detects leaks of objects that should be destroyed very close to
sandbox exit. The old checker very rarely detected these leaks, because it
relied on garbage collection to be run in a short window of time.
- Panics if we forgot to enable leak check on a ref-counted object (we will try
to remove it from the map when it is destroyed, but it will never have been
added).
- Can store extra logging information in the map values without adding to the
size of the ref count struct itself. With the size of just an int64, the ref
count object remains compact, meaning frequent operations like IncRef/DecRef
are more cache-efficient.
- Can aggregate leak results in a single report after the sandbox exits.
Instead of having warnings littered in the log, which were
non-deterministically triggered by garbage collection, we can print all
warning messages at once. Note that this could also be a limitation--the
sandbox must exit properly for leaks to be detected.
Some basic benchmarking indicates that this change does not significantly
affect performance when leak checking is enabled, which is understandable
since registering/unregistering is only done once for each filesystem object.
Updates #1486.
PiperOrigin-RevId: 338685972
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This uses the refs_vfs2 template in vfs2 as well as objects common to vfs1 and
vfs2. Note that vfs1-only refcounts are not replaced, since vfs1 will be deleted
soon anyway.
The following structs now use the new tool, with leak check enabled:
devpts:rootInode
fuse:inode
kernfs:Dentry
kernfs:dir
kernfs:readonlyDir
kernfs:StaticDirectory
proc:fdDirInode
proc:fdInfoDirInode
proc:subtasksInode
proc:taskInode
proc:tasksInode
vfs:FileDescription
vfs:MountNamespace
vfs:Filesystem
sys:dir
kernel:FSContext
kernel:ProcessGroup
kernel:Session
shm:Shm
mm:aioMappable
mm:SpecialMappable
transport:queue
And the following use the template, but because they currently are not leak
checked, a TODO is left instead of enabling leak check in this patch:
kernel:FDTable
tun:tunEndpoint
Updates #1486.
PiperOrigin-RevId: 328460377
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The abstract socket namespace no longer holds any references on sockets.
Instead, TryIncRef() is used when a socket is being retrieved in
BoundEndpoint(). Abstract sockets are now responsible for removing themselves
from the namespace they are in, when they are destroyed.
Updates #1486.
PiperOrigin-RevId: 327064173
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Updates #1486.
PiperOrigin-RevId: 326354750
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The utility has several differences from the VFS1 equivalent:
- There are no weak references, which have a significant overhead
- In order to print useful debug messages with the type of the reference-
counted object, we use a generic Refs object with the owner type as a
template parameter. In vfs1, this was accomplished by storing a type name
and caller stack directly in the ref count (as in vfs1), which increases the
struct size by 6x. (Note that the caller stack was needed because fs types
like Dirent were shared by all fs implementations; in vfs2, each impl has
its own data structures, so this is no longer necessary.)
Updates #1486.
PiperOrigin-RevId: 325271469
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PiperOrigin-RevId: 324931854
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The utility has several differences from the VFS1 equivalent:
- There are no weak references, which have a significant overhead
- In order to print useful debug messages with the type of the reference-
counted object, we use a generic Refs object with the owner type as a
template parameter. In vfs1, this was accomplished by storing a type name
and caller stack directly in the ref count (as in vfs1), which increases the
struct size by 6x. (Note that the caller stack was needed because fs types
like Dirent were shared by all fs implementations; in vfs2, each impl has
its own data structures, so this is no longer necessary.)
As an example, the utility is added to tmpfs.inode.
Updates #1486.
PiperOrigin-RevId: 324906582
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