[release-branch.go1.23] runtime: set mspan limit field early and eagerly

Currently the mspan limit field is set after allocSpan returns, *after*
the span has already been published to the GC (including the
conservative scanner). But the limit field is load-bearing, because it's
checked to filter out invalid pointers. A stale limit value could cause
a crash by having the conservative scanner access allocBits out of
bounds.

Fix this by setting the mspan limit field before publishing the span.
For large objects and arena chunks, we adjust the limit down after
allocSpan because we don't have access to the true object's size from
allocSpan. However this is safe, since we first initialize the limit to
something definitely safe (the actual span bounds) and only adjust it
down after. Adjusting it down has the benefit of more precise debug
output, but the window in which it's imprecise is also fine because a
single object (logically, with arena chunks) occupies the whole span, so
the 'invalid' part of the memory will just safely point back to that
object. We can't do this for smaller objects because the limit will
include space that does *not* contain any valid objects.

For #74288.
Fixes #74289.

Change-Id: I0a22e5b9bccc1bfdf51d2b73ea7130f1b99c0c7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/682655
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit 6bbe5e56d0b4957e0204b464bfd76768b13c9617)
Reviewed-on: https://go-review.googlesource.com/c/go/+/684096
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

diff --git a/src/runtime/arena.go b/src/runtime/arena.go
index cd9a9dfae10abc240ccbd7783344cd1acdbaafb1..ab81a8dd704d327b7fe642837ee05f3669634c3e 100644 (file)
--- a/src/runtime/arena.go
+++ b/src/runtime/arena.go
@@ -1063,10 +1063,18 @@ func (h *mheap) allocUserArenaChunk() *mspan {
        h.initSpan(s, spanAllocHeap, spc, base, userArenaChunkPages)
        s.isUserArenaChunk = true
        s.elemsize -= userArenaChunkReserveBytes()
-       s.limit = s.base() + s.elemsize
        s.freeindex = 1
        s.allocCount = 1
 
+       // Adjust s.limit down to the object-containing part of the span.
+       //
+       // This is just to create a slightly tighter bound on the limit.
+       // It's totally OK if the garbage collector, in particular
+       // conservative scanning, can temporarily observes an inflated
+       // limit. It will simply mark the whole chunk or just skip it
+       // since we're in the mark phase anyway.
+       s.limit = s.base() + s.elemsize
+
        // Account for this new arena chunk memory.
        gcController.heapInUse.add(int64(userArenaChunkBytes))
        gcController.heapReleased.add(-int64(userArenaChunkBytes))

diff --git a/src/runtime/mcache.go b/src/runtime/mcache.go
index e8da133a69490e1f274c581f667c493d43d694c5..e28dbb020193322d836c4dd10753f26f2175bf9d 100644 (file)
--- a/src/runtime/mcache.go
+++ b/src/runtime/mcache.go
@@ -251,6 +251,14 @@ func (c *mcache) allocLarge(size uintptr, noscan bool) *mspan {
        // Put the large span in the mcentral swept list so that it's
        // visible to the background sweeper.
        mheap_.central[spc].mcentral.fullSwept(mheap_.sweepgen).push(s)
+
+       // Adjust s.limit down to the object-containing part of the span.
+       //
+       // This is just to create a slightly tighter bound on the limit.
+       // It's totally OK if the garbage collector, in particular
+       // conservative scanning, can temporarily observes an inflated
+       // limit. It will simply mark the whole object or just skip it
+       // since we're in the mark phase anyway.
        s.limit = s.base() + size
        s.initHeapBits(false)
        return s

diff --git a/src/runtime/mcentral.go b/src/runtime/mcentral.go
index bf597e1936da49575cb71f2017f5a042ff035d58..28c57eb30b86724430a87a28ef216e3d3e7bd7be 100644 (file)
--- a/src/runtime/mcentral.go
+++ b/src/runtime/mcentral.go
@@ -249,17 +249,10 @@ func (c *mcentral) uncacheSpan(s *mspan) {
 // grow allocates a new empty span from the heap and initializes it for c's size class.
 func (c *mcentral) grow() *mspan {
        npages := uintptr(class_to_allocnpages[c.spanclass.sizeclass()])
-       size := uintptr(class_to_size[c.spanclass.sizeclass()])
-
        s := mheap_.alloc(npages, c.spanclass)
        if s == nil {
                return nil
        }
-
-       // Use division by multiplication and shifts to quickly compute:
-       // n := (npages << _PageShift) / size
-       n := s.divideByElemSize(npages << _PageShift)
-       s.limit = s.base() + size*n
        s.initHeapBits(false)
        return s
 }

diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index bfca2d105b742656d9bd3d7aed1281112ab5211d..b27901cedc155787e3064da0f71ef0a03deaa420 100644 (file)
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -1390,7 +1390,6 @@ func (h *mheap) initSpan(s *mspan, typ spanAllocType, spanclass spanClass, base,
        if typ.manual() {
                s.manualFreeList = 0
                s.nelems = 0
-               s.limit = s.base() + s.npages*pageSize
                s.state.set(mSpanManual)
        } else {
                // We must set span properties before the span is published anywhere
@@ -1418,6 +1417,9 @@ func (h *mheap) initSpan(s *mspan, typ spanAllocType, spanclass spanClass, base,
                s.gcmarkBits = newMarkBits(uintptr(s.nelems))
                s.allocBits = newAllocBits(uintptr(s.nelems))
 
+               // Adjust s.limit down to the object-containing part of the span.
+               s.limit = s.base() + uintptr(s.elemsize)*uintptr(s.nelems)
+
                // It's safe to access h.sweepgen without the heap lock because it's
                // only ever updated with the world stopped and we run on the
                // systemstack which blocks a STW transition.
@@ -1701,6 +1703,7 @@ func (span *mspan) init(base uintptr, npages uintptr) {
        span.list = nil
        span.startAddr = base
        span.npages = npages
+       span.limit = base + npages*pageSize // see go.dev/issue/74288; adjusted later for heap spans
        span.allocCount = 0
        span.spanclass = 0
        span.elemsize = 0