[compiler-rt] [memprof] Move allocator base to avoid conflict with high-entropy ASLR (PR #85834)

[Thurston Dang via llvm-commits]

https://github.com/thurstond updated https://github.com/llvm/llvm-project/pull/85834

>From 5e94620ced5cad0fea26125e4938c5c4d21b8e68 Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston at google.com>
Date: Tue, 19 Mar 2024 17:51:11 +0000
Subject: [PATCH 1/2] [memprof] Move allocator base to avoid conflict with
 high-entropy ASLR

memprof often fails when ASLR entropy is too high ('sudo sysctl vm.mmap_rnd_bits=32; ninja check-memprof'),
which is the default setting for newer versions of Ubuntu (https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/jammy/commit/?h=hwe-6.5-next--2024.03.04-1--auto&id=6b522637c6a7dabd8530026ae933fb5ff17e877f). This patch
fixes the issue by moving the allocator base, analogously to ASan (https://reviews.llvm.org/D148280).

Explanation from the ASan patch: when CONFIG_ARCH_MMAP_RND_BITS == 32,
it will frequently conflict with memprof's allocator, because the
PIE program segment base address of 0x555555555554 plus an ASLR shift of up to
((2**32) * 4K == 0x100000000000) will sometimes exceed memprof's hardcoded
base address of 0x600000000000. We fix this by simply moving the allocator base
to 0x500000000000, which is below the PIE program segment base address. This is
cleaner than trying to move it to another location that is sandwiched between
the PIE program and library segments, because if either of those grow too large,
it will collide with the allocator region.

Note that we will never need to change this base address again (unless we want to increase
the size of the allocator), because ASLR cannot be set above 32-bits for x86-64 Linux (the
PIE program segment and library segments would collide with each other; see also
ARCH_MMAP_RND_BITS_MAX in https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig).
---
 compiler-rt/lib/memprof/memprof_allocator.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler-rt/lib/memprof/memprof_allocator.h b/compiler-rt/lib/memprof/memprof_allocator.h
index f583cee020e4ff..757a44babc352e 100644
--- a/compiler-rt/lib/memprof/memprof_allocator.h
+++ b/compiler-rt/lib/memprof/memprof_allocator.h
@@ -46,7 +46,7 @@ struct MemprofMapUnmapCallback {
   void OnUnmap(uptr p, uptr size) const;
 };
 
-constexpr uptr kAllocatorSpace = 0x600000000000ULL;
+constexpr uptr kAllocatorSpace = 0x500000000000ULL;
 constexpr uptr kAllocatorSize = 0x40000000000ULL; // 4T.
 typedef DefaultSizeClassMap SizeClassMap;
 template <typename AddressSpaceViewTy>

>From 95b8b26090be1217c7bb206b17b4573e36a159b9 Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston at google.com>
Date: Tue, 19 Mar 2024 18:01:29 +0000
Subject: [PATCH 2/2] Keep the old allocator location for the Fruitman, just in
 case

---
 compiler-rt/lib/memprof/memprof_allocator.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler-rt/lib/memprof/memprof_allocator.h b/compiler-rt/lib/memprof/memprof_allocator.h
index 757a44babc352e..89e924f80d41ad 100644
--- a/compiler-rt/lib/memprof/memprof_allocator.h
+++ b/compiler-rt/lib/memprof/memprof_allocator.h
@@ -46,7 +46,11 @@ struct MemprofMapUnmapCallback {
   void OnUnmap(uptr p, uptr size) const;
 };
 
+#if SANITIZER_APPLE
+constexpr uptr kAllocatorSpace = 0x600000000000ULL;
+#else
 constexpr uptr kAllocatorSpace = 0x500000000000ULL;
+#endif
 constexpr uptr kAllocatorSize = 0x40000000000ULL; // 4T.
 typedef DefaultSizeClassMap SizeClassMap;
 template <typename AddressSpaceViewTy>