[gwp-asan] Initialize AllocatorVersionMagic at runtime

[cryptoad]

Summary

GWP-ASan's AllocatorState was recently extended with a
AllocatorVersionMagic structure required so that GWP-ASan bug reports
can be understood by tools at different versions.

On Fuchsia, this in included in the scudo::Allocator structure, and
by having non-zero initializers, this effectively moved the static
allocator structure from the .bss segment to the .data segment, thus
increasing (significantly) the size of the libc.

This CL proposes to initialize the structure with its magic numbers at
runtime, allowing for the allocator to go back into the .bss segment.

I will work on adding a test on the Scudo side to ensure that this type
of changes get detected early on. Additional work is also needed to
reduce the footprint of the (large) memory-tagging related structures
that are currently part of the allocator.

[hctim]

compiler-rt/lib/gwp_asan/common.h

44

Can you hoist these inside the struct (next to their declarations), and add a note that the values are copied into the struct at runtime, during GuardedPoolAllocator::initialize() so that GWP-ASan remains completely in the .bss segment?

[cryptoad]

Addressing Mitch's request.

[hctim]

LGTM

compiler-rt/lib/gwp_asan/common.h

 
 #include <stddef.h>
 #include <stdint.h>
 
 namespace gwp_asan {
 
 // Magic header that resides in the AllocatorState so that GWP-ASan bugreports
 // can be understood by tools at different versions. Out-of-process crash
-// handlers, like crashpad on Fuchsia, take the raw conents of the
+// handlers, like crashpad on Fuchsia, take the raw contents of the
 // AllocationMetatada array and the AllocatorState, and shove them into the
 // minidump. Online unpacking of these structs needs to know from which version
-// of GWP-ASan its extracting the information, as the structures are not stable.
+// of GWP-ASan it's extracting the information, as the structures are not
+// stable.
 struct AllocatorVersionMagic {
-  const uint8_t Magic[4] = {'A', 'S', 'A', 'N'};
+  // The values are copied into the structure at runtime, during
+  // `GuardedPoolAllocator::init()` so that GWP-ASan remains completely in the
+  // `.bss` segment.
+  static constexpr uint8_t kAllocatorVersionMagic[4] = {'A', 'S', 'A', 'N'};
+  uint8_t Magic[4] = {};
   // Update the version number when the AllocatorState or AllocationMetadata
   // change.
-  const uint16_t Version = 1;
-  const uint16_t Reserved = 0;
+  static constexpr uint16_t kAllocatorVersion = 1;
+  uint16_t Version = 0;
+  uint16_t Reserved = 0;
 };
 
 enum class Error : uint8_t {
   UNKNOWN,
   USE_AFTER_FREE,
   DOUBLE_FREE,
   INVALID_FREE,
   BUFFER_OVERFLOW,
   BUFFER_UNDERFLOW
 };
 
 const char *ErrorToString(const Error &E);
struct AllocationMetadata {
   bool IsDeallocated = false;
 };
 
 // This holds the state that's shared between the GWP-ASan allocator and the
 // crash handler. This, in conjunction with the Metadata array, forms the entire
 // set of information required for understanding a GWP-ASan crash.
 struct AllocatorState {
   constexpr AllocatorState() {}
-  const AllocatorVersionMagic VersionMagic{};
+  AllocatorVersionMagic VersionMagic{};
 
   // Returns whether the provided pointer is a current sampled allocation that
   // is owned by this pool.
   GWP_ASAN_ALWAYS_INLINE bool pointerIsMine(const void *Ptr) const {
     uintptr_t P = reinterpret_cast<uintptr_t>(Ptr);
     return P < GuardedPagePoolEnd && GuardedPagePool <= P;
   }
 

compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp

void GuardedPoolAllocator::init(const options::Options &Opts) {
   Check(Opts.SampleRate >= 0, "GWP-ASan Error: SampleRate is < 0.");
   Check(Opts.SampleRate < (1 << 30), "GWP-ASan Error: SampleRate is >= 2^30.");
   Check(Opts.MaxSimultaneousAllocations >= 0,
         "GWP-ASan Error: MaxSimultaneousAllocations is < 0.");
 
   SingletonPtr = this;
   Backtrace = Opts.Backtrace;
 
+  State.VersionMagic = {{AllocatorVersionMagic::kAllocatorVersionMagic[0],
+                         AllocatorVersionMagic::kAllocatorVersionMagic[1],
+                         AllocatorVersionMagic::kAllocatorVersionMagic[2],
+                         AllocatorVersionMagic::kAllocatorVersionMagic[3]},
+                        AllocatorVersionMagic::kAllocatorVersion,
+                        0};
+
   State.MaxSimultaneousAllocations = Opts.MaxSimultaneousAllocations;
 
   const size_t PageSize = getPlatformPageSize();
   // getPageAddr() and roundUpTo() assume the page size to be a power of 2.
   assert((PageSize & (PageSize - 1)) == 0);
   State.PageSize = PageSize;
 
   size_t PoolBytesRequired =