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Wave 2 package X1. Three CI split-subset traces died on the Redmi under inproc and
passed under monolith on the same device and APK, on the client thread, in the
replayer's memcpy into a pointer MobileGL had handed it: SIGSEGV / SEGV_ACCERR on the
destination's FIRST byte, destination page-aligned.
The tombstone names the same address twice, in two different address spaces:
x0 b4000073ac9a5000 ... x2 0000000001800000
signal 11 (SIGSEGV), code 2 (SEGV_ACCERR), fault addr 0x00000073ac9a5000
x0 is the pointer the application writes through, carrying bionic's 0xb4 heap tag
(the same tombstone reports tagged_addr_ctrl PR_TAGGED_ADDR_ENABLE). fault addr is
si_addr, which the kernel reports UNTAGGED.
TrackWriteMap took shadowBase from MappedData() - a tagged heap pointer - kept the tag
through the page masking, mprotect()ed the mapped range PROT_READ and published the
TAGGED base/end into the slot. The handler compared si_addr against it:
if (base <= kSlotSettingUp || address < base) continue;
0x73ac9a5000 < 0xb4000073ac9a5000 is true for every fault, forever. Ownership test (2)
never matched, and every fault on a page the tracker had itself protected was chained
to debuggerd. mprotect was fine throughout - do_mprotect_pkey untags its own argument -
so the right pages really were read-only; only the recognition was broken. x86-64 has
no top-byte tag, which is why every host lane has always been green.
The fix is two rows and one ordering:
* UntagAddress(), applied to the shadow base at registration, to the shadow base in
PushBlocksFor (so trackedBase - shadowBase stays in one space) and to si_addr in
the handler. Written WITHOUT an #if defined(__aarch64__): masking bits 56-63 is
the identity for every valid userspace address off arm64, and an arch guard would
have put the only gate that can catch this on the one platform CI never runs.
Dereferences keep the tagged pointer they came from.
* SlotOwnsFault(), the ownership row the handler and the unit case both read.
* REGISTRATION ARMS NOTHING. AcquireMemoryRange registered and only then returned
Bytes() + range.start, so glMapBufferRange handed the application a PROT_READ
pointer whose writability depended on a handler reaching the writing thread - and
the application is free to write a mapped arena from a worker that blocks every
signal. Nothing is lost by waiting: every page bit is already set at registration,
so the first push ships the whole interior regardless, and it is that push which
arms the pages. 6144 pointless faults per 24 MB arena go with it, and the
invariant the file rests on (bit set <=> page writable) is now true from
registration onward instead of only from the first push.
Diagnostic, independent of the fix: one MGLOG_I_ONCE at the first registration naming
the mapping the way the device run needed and did not have (allocator, pointer, ITS
TAG, extent, range, published span, pages, alignment arm), and a decline record in the
handler - relaxed atomic stores plus one async-signal-safe write(2) - readable through
DeclinedFaults(). A non-zero decline count whose si_addr lies inside nearest-tracked
once the top byte is masked off is this defect's signature and nothing else's.
Not the mechanism, and checked: libsigchain (the tombstone's "F libc : Fatal signal 11"
is bionic's own handler, so the chain ran), the 16 KiB-page refusal (the device is a
4 KiB kernel and the protection demonstrably took), RefaultOfANonWriteAccess (matched
is false long before it is consulted), and the P5d shadow allocator (correct - it is
what makes the destination page-aligned in the tombstone).
Red-once, two and orthogonal (MG_Test/Wire/PersistentMapFaultTest.cpp, 6 cases):
dropping the two UntagAddress calls in SlotOwnsFault reds only
AKernelsUntaggedFaultAddressIsOwnedByATaggedShadowsSlot, which runs the device's own
two addresses through the real predicate on an x86-64 host; restoring the mprotect at
the end of TrackWriteMap reds only AMapHandsBackARangeWritableWithoutTheFaultHandler
(the child dies by SIGSEGV) and RegistrationLeavesEveryPageOfTheRangeReadWrite.
AnArmedPageIsStillAnsweredByTheHandlerAndMadeWritable is the anti-vacuity case.
Gates: ctest -L unit 2408/2408; integration-split 186/186; integration-spawn 102/102;
integration-magma-split 73/73 (PersistentCoherentMapScenario green in the Split, Spawn,
PersistentMapArm and SmallRing variants); fatal_census.py 0 unmarked. G1 holds - pull
.text 10822147 = 0xa52203 unchanged, symbol report 0 added / 0 removed / 0 resized /
0 renamed, nm --defined-only identical at 30570 and nm -D at 11933. Every line is under
MOBILEGL_BUILD_DISAGGREGATED (CMakeLists.txt:592-659), so the pull build sees none of it.
Evidence and the device experiment: docs/Disaggregated/notes/p7/x1-persistent-map-segv.md