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[Darwin] Fix symbolization for recent simulator runtimes.
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Authored by delcypher on Apr 14 2020, 10:27 PM.

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Description

[Darwin] Fix symbolization for recent simulator runtimes.

Summary:
Due to sandbox restrictions in the recent versions of the simulator runtime the
atos program is no longer able to access the task port of a parent process
without additional help.

This patch fixes this by registering a task port for the parent process
before spawning atos and also tells atos to look for this by setting
a special environment variable.

This patch is based on an Apple internal fix (rdar://problem/43693565) that
unfortunately contained a bug (rdar://problem/58789439) because it used
setenv() to set the special environment variable. This is not safe because in
certain circumstances this can trigger a call to realloc() which can fail
during symbolization leading to deadlock. A test case is included that captures
this problem.

The approach used to set the necessary environment variable is as
follows:

  1. Calling putenv() early during process init (but late enough that

malloc/realloc works) to set a dummy value for the environment variable.

  1. Just before atos is spawned the storage for the environment

variable is modified to contain the correct PID.

A flaw with this approach is that if the application messes with the
atos environment variable (i.e. unsets it or changes it) between the
time its set and the time we need it then symbolization will fail. We
will ignore this issue for now but a DCHECK() is included in the patch
that documents this assumption but doesn't check it at runtime to avoid
calling getenv().

The issue reported in rdar://problem/58789439 manifested as a deadlock
during symbolization in the following situation:

  1. Before TSan detects an issue something outside of the runtime calls

setenv() that sets a new environment variable that wasn't previously
set. This triggers a call to malloc() to allocate a new environment
array. This uses TSan's normal user-facing allocator. LibC stores this
pointer for future use later.

  1. TSan detects an issue and tries to launch the symbolizer. When we are in the

symbolizer we switch to a different (internal allocator) and then we call
setenv() to set a new environment variable. When this happen setenv() sees
that it needs to make the environment array larger and calls realloc() on the
existing enviroment array because it remembers that it previously allocated
memory for it. Calling realloc() fails here because it is being called on a
pointer its never seen before.

The included test case closely reproduces the originally reported
problem but it doesn't replicate the `((kBlockMagic)) ==
((((u64*)addr)[0])` assertion failure exactly. This is due to the way
TSan's normal allocator allocates the environment array the first time
it is allocated. In the test program addr[0] accesses an inaccessible
page and raises SIGBUS. If TSan's SIGBUS signal handler is active, the
signal is caught and symbolication is attempted again which results in
deadlock.

In the originally reported problem the pointer is successfully derefenced but
then the assert fails due to the provided pointer not coming from the active
allocator. When the assert fails TSan tries to symbolicate the stacktrace while
already being in the middle of symbolication which results in deadlock.

rdar://problem/58789439

Reviewers: kubamracek, yln

Subscribers: jfb, #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D78179

(cherry picked from commit 861b69faee5df8d4e13ef316c7474a10e4069e81)