diff options
Diffstat (limited to 'pkg/sentry/kernel/shm')
| -rw-r--r-- | pkg/sentry/kernel/shm/BUILD | 9 | ||||
| -rw-r--r-- | pkg/sentry/kernel/shm/shm.go | 106 |
2 files changed, 74 insertions, 41 deletions
diff --git a/pkg/sentry/kernel/shm/BUILD b/pkg/sentry/kernel/shm/BUILD index cd48945e6..c211fc8d0 100644 --- a/pkg/sentry/kernel/shm/BUILD +++ b/pkg/sentry/kernel/shm/BUILD @@ -1,4 +1,4 @@ -load("//tools/go_stateify:defs.bzl", "go_library") +load("//tools:defs.bzl", "go_library") package(licenses = ["notice"]) @@ -8,22 +8,21 @@ go_library( "device.go", "shm.go", ], - importpath = "gvisor.dev/gvisor/pkg/sentry/kernel/shm", visibility = ["//pkg/sentry:internal"], deps = [ "//pkg/abi/linux", + "//pkg/context", "//pkg/log", "//pkg/refs", - "//pkg/sentry/context", "//pkg/sentry/device", "//pkg/sentry/fs", "//pkg/sentry/kernel/auth", "//pkg/sentry/kernel/time", "//pkg/sentry/memmap", "//pkg/sentry/pgalloc", - "//pkg/sentry/platform", "//pkg/sentry/usage", - "//pkg/sentry/usermem", + "//pkg/sync", "//pkg/syserror", + "//pkg/usermem", ], ) diff --git a/pkg/sentry/kernel/shm/shm.go b/pkg/sentry/kernel/shm/shm.go index 5bd610f68..13ec7afe0 100644 --- a/pkg/sentry/kernel/shm/shm.go +++ b/pkg/sentry/kernel/shm/shm.go @@ -35,21 +35,20 @@ package shm import ( "fmt" - "sync" "gvisor.dev/gvisor/pkg/abi/linux" + "gvisor.dev/gvisor/pkg/context" "gvisor.dev/gvisor/pkg/log" "gvisor.dev/gvisor/pkg/refs" - "gvisor.dev/gvisor/pkg/sentry/context" "gvisor.dev/gvisor/pkg/sentry/fs" "gvisor.dev/gvisor/pkg/sentry/kernel/auth" ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time" "gvisor.dev/gvisor/pkg/sentry/memmap" "gvisor.dev/gvisor/pkg/sentry/pgalloc" - "gvisor.dev/gvisor/pkg/sentry/platform" "gvisor.dev/gvisor/pkg/sentry/usage" - "gvisor.dev/gvisor/pkg/sentry/usermem" + "gvisor.dev/gvisor/pkg/sync" "gvisor.dev/gvisor/pkg/syserror" + "gvisor.dev/gvisor/pkg/usermem" ) // Key represents a shm segment key. Analogous to a file name. @@ -71,9 +70,20 @@ type Registry struct { mu sync.Mutex `state:"nosave"` // shms maps segment ids to segments. + // + // shms holds all referenced segments, which are removed on the last + // DecRef. Thus, it cannot itself hold a reference on the Shm. + // + // Since removal only occurs after the last (unlocked) DecRef, there + // exists a short window during which a Shm still exists in Shm, but is + // unreferenced. Users must use TryIncRef to determine if the Shm is + // still valid. shms map[ID]*Shm // keysToShms maps segment keys to segments. + // + // Shms in keysToShms are guaranteed to be referenced, as they are + // removed by disassociateKey before the last DecRef. keysToShms map[Key]*Shm // Sum of the sizes of all existing segments rounded up to page size, in @@ -95,10 +105,18 @@ func NewRegistry(userNS *auth.UserNamespace) *Registry { } // FindByID looks up a segment given an ID. +// +// FindByID returns a reference on Shm. func (r *Registry) FindByID(id ID) *Shm { r.mu.Lock() defer r.mu.Unlock() - return r.shms[id] + s := r.shms[id] + // Take a reference on s. If TryIncRef fails, s has reached the last + // DecRef, but hasn't quite been removed from r.shms yet. + if s != nil && s.TryIncRef() { + return s + } + return nil } // dissociateKey removes the association between a segment and its key, @@ -119,6 +137,8 @@ func (r *Registry) dissociateKey(s *Shm) { // FindOrCreate looks up or creates a segment in the registry. It's functionally // analogous to open(2). +// +// FindOrCreate returns a reference on Shm. func (r *Registry) FindOrCreate(ctx context.Context, pid int32, key Key, size uint64, mode linux.FileMode, private, create, exclusive bool) (*Shm, error) { if (create || private) && (size < linux.SHMMIN || size > linux.SHMMAX) { // "A new segment was to be created and size is less than SHMMIN or @@ -166,6 +186,7 @@ func (r *Registry) FindOrCreate(ctx context.Context, pid int32, key Key, size ui return nil, syserror.EEXIST } + shm.IncRef() return shm, nil } @@ -193,7 +214,14 @@ func (r *Registry) FindOrCreate(ctx context.Context, pid int32, key Key, size ui // Need to create a new segment. creator := fs.FileOwnerFromContext(ctx) perms := fs.FilePermsFromMode(mode) - return r.newShm(ctx, pid, key, creator, perms, size) + s, err := r.newShm(ctx, pid, key, creator, perms, size) + if err != nil { + return nil, err + } + // The initial reference is held by s itself. Take another to return to + // the caller. + s.IncRef() + return s, nil } // newShm creates a new segment in the registry. @@ -296,22 +324,26 @@ func (r *Registry) remove(s *Shm) { // Shm represents a single shared memory segment. // -// Shm segment are backed directly by an allocation from platform -// memory. Segments are always mapped as a whole, greatly simplifying how -// mappings are tracked. However note that mremap and munmap calls may cause the -// vma for a segment to become fragmented; which requires special care when -// unmapping a segment. See mm/shm.go. +// Shm segment are backed directly by an allocation from platform memory. +// Segments are always mapped as a whole, greatly simplifying how mappings are +// tracked. However note that mremap and munmap calls may cause the vma for a +// segment to become fragmented; which requires special care when unmapping a +// segment. See mm/shm.go. // // Segments persist until they are explicitly marked for destruction via -// shmctl(SHM_RMID). +// MarkDestroyed(). // // Shm implements memmap.Mappable and memmap.MappingIdentity. // // +stateify savable type Shm struct { - // AtomicRefCount tracks the number of references to this segment from - // maps. A segment always holds a reference to itself, until it's marked for + // AtomicRefCount tracks the number of references to this segment. + // + // A segment holds a reference to itself until it is marked for // destruction. + // + // In addition to direct users, the MemoryManager will hold references + // via MappingIdentity. refs.AtomicRefCount mfp pgalloc.MemoryFileProvider @@ -337,7 +369,7 @@ type Shm struct { // fr is the offset into mfp.MemoryFile() that backs this contents of this // segment. Immutable. - fr platform.FileRange + fr memmap.FileRange // mu protects all fields below. mu sync.Mutex `state:"nosave"` @@ -399,8 +431,8 @@ func (s *Shm) InodeID() uint64 { // DecRef overrides refs.RefCount.DecRef with a destructor. // // Precondition: Caller must not hold s.mu. -func (s *Shm) DecRef() { - s.DecRefWithDestructor(s.destroy) +func (s *Shm) DecRef(ctx context.Context) { + s.DecRefWithDestructor(ctx, s.destroy) } // Msync implements memmap.MappingIdentity.Msync. Msync is a no-op for shm @@ -428,7 +460,7 @@ func (s *Shm) AddMapping(ctx context.Context, _ memmap.MappingSpace, _ usermem.A func (s *Shm) RemoveMapping(ctx context.Context, _ memmap.MappingSpace, _ usermem.AddrRange, _ uint64, _ bool) { s.mu.Lock() defer s.mu.Unlock() - // TODO(b/38173783): RemoveMapping may be called during task exit, when ctx + // RemoveMapping may be called during task exit, when ctx // is context.Background. Gracefully handle missing clocks. Failing to // update the detach time in these cases is ok, since no one can observe the // omission. @@ -484,9 +516,8 @@ type AttachOpts struct { // ConfigureAttach creates an mmap configuration for the segment with the // requested attach options. // -// ConfigureAttach returns with a ref on s on success. The caller should drop -// this once the map is installed. This reference prevents s from being -// destroyed before the returned configuration is used. +// Postconditions: The returned MMapOpts are valid only as long as a reference +// continues to be held on s. func (s *Shm) ConfigureAttach(ctx context.Context, addr usermem.Addr, opts AttachOpts) (memmap.MMapOpts, error) { s.mu.Lock() defer s.mu.Unlock() @@ -504,7 +535,6 @@ func (s *Shm) ConfigureAttach(ctx context.Context, addr usermem.Addr, opts Attac // in the user namespace that governs its IPC namespace." - man shmat(2) return memmap.MMapOpts{}, syserror.EACCES } - s.IncRef() return memmap.MMapOpts{ Length: s.size, Offset: 0, @@ -549,10 +579,15 @@ func (s *Shm) IPCStat(ctx context.Context) (*linux.ShmidDS, error) { } creds := auth.CredentialsFromContext(ctx) - nattach := uint64(s.ReadRefs()) - // Don't report the self-reference we keep prior to being marked for - // destruction. However, also don't report a count of -1 for segments marked - // as destroyed, with no mappings. + // Use the reference count as a rudimentary count of the number of + // attaches. We exclude: + // + // 1. The reference the caller holds. + // 2. The self-reference held by s prior to destruction. + // + // Note that this may still overcount by including transient references + // used in concurrent calls. + nattach := uint64(s.ReadRefs()) - 1 if !s.pendingDestruction { nattach-- } @@ -607,7 +642,7 @@ func (s *Shm) Set(ctx context.Context, ds *linux.ShmidDS) error { return nil } -func (s *Shm) destroy() { +func (s *Shm) destroy(context.Context) { s.mfp.MemoryFile().DecRef(s.fr) s.registry.remove(s) } @@ -616,22 +651,21 @@ func (s *Shm) destroy() { // destroyed once it has no references. MarkDestroyed may be called multiple // times, and is safe to call after a segment has already been destroyed. See // shmctl(IPC_RMID). -func (s *Shm) MarkDestroyed() { +func (s *Shm) MarkDestroyed(ctx context.Context) { s.registry.dissociateKey(s) s.mu.Lock() - // Only drop the segment's self-reference once, when destruction is - // requested. Otherwise, repeated calls to shmctl(IPC_RMID) would force a - // segment to be destroyed prematurely, potentially with active maps to the - // segment's address range. Remaining references are dropped when the - // segment is detached or unmaped. + defer s.mu.Unlock() if !s.pendingDestruction { s.pendingDestruction = true - s.mu.Unlock() // Must release s.mu before calling s.DecRef. - s.DecRef() + // Drop the self-reference so destruction occurs when all + // external references are gone. + // + // N.B. This cannot be the final DecRef, as the caller also + // holds a reference. + s.DecRef(ctx) return } - s.mu.Unlock() } // checkOwnership verifies whether a segment may be accessed by ctx as an |