From c8d1e4cf833a2d36bfabda5e864a7cb357bc8b78 Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Wed, 26 Feb 2020 15:12:33 -0500 Subject: [PATCH] [release-branch.go1.14] runtime: fix rounding in materializeGCProg materializeGCProg allocates a temporary buffer for unrolling a GC program. Unfortunately, when computing the size of the buffer, it rounds *down* the number of bytes needed to store bitmap before rounding up the number of pages needed to store those bytes. The fact that it rounds up to pages usually mitigates the rounding down, but the type from #37470 exists right on the boundary where this doesn't work: type Sequencer struct { htable [1 << 17]uint32 buf []byte } On 64-bit, this GC bitmap is exactly 8 KiB of zeros, followed by three one bits. Hence, this needs 8193 bytes of storage, but the current math in materializeGCProg rounds *down* the three one bits to 8192 bytes. Since this is exactly pageSize, the next step of rounding up to the page size doesn't mitigate this error, and materializeGCProg allocates a buffer that is one byte too small. runGCProg then writes one byte past the end of this buffer, causing either a segfault (if you're lucky!) or memory corruption. Updates #37470. Fixes #37480. Change-Id: Iad24c463c501cd9b1dc1924bc2ad007991a094a0 Reviewed-on: https://go-review.googlesource.com/c/go/+/224417 Run-TryBot: Austin Clements Reviewed-by: Cherry Zhang TryBot-Result: Gobot Gobot --- src/runtime/mbitmap.go | 6 +++++- src/runtime/stubs.go | 7 +++++++ 2 files changed, 12 insertions(+), 1 deletion(-) diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go index 55c0282403..2045789fa4 100644 --- a/src/runtime/mbitmap.go +++ b/src/runtime/mbitmap.go @@ -1921,7 +1921,11 @@ Run: // The bitmask starts at s.startAddr. // The result must be deallocated with dematerializeGCProg. func materializeGCProg(ptrdata uintptr, prog *byte) *mspan { - s := mheap_.allocManual((ptrdata/(8*sys.PtrSize)+pageSize-1)/pageSize, &memstats.gc_sys) + // Each word of ptrdata needs one bit in the bitmap. + bitmapBytes := divRoundUp(ptrdata, 8*sys.PtrSize) + // Compute the number of pages needed for bitmapBytes. + pages := divRoundUp(bitmapBytes, pageSize) + s := mheap_.allocManual(pages, &memstats.gc_sys) runGCProg(addb(prog, 4), nil, (*byte)(unsafe.Pointer(s.startAddr)), 1) return s } diff --git a/src/runtime/stubs.go b/src/runtime/stubs.go index b8d4d6b30a..2c6f027c2c 100644 --- a/src/runtime/stubs.go +++ b/src/runtime/stubs.go @@ -310,6 +310,13 @@ func alignDown(n, a uintptr) uintptr { return n &^ (a - 1) } +// divRoundUp returns ceil(n / a). +func divRoundUp(n, a uintptr) uintptr { + // a is generally a power of two. This will get inlined and + // the compiler will optimize the division. + return (n + a - 1) / a +} + // checkASM reports whether assembly runtime checks have passed. func checkASM() bool -- 2.48.1