[scudo] Always MAP_NORESERVE in to release pages

[vitalybuka]

Summary

This uses QEMU-user workaround as
default behaviour.

[vitalybuka]

rebase

[vitalybuka]

rebase

[hctim]

LGTM. I wouldn't expect any perf diff here, but it might be worth checking.

compiler-rt/lib/scudo/standalone/linux.cpp

 
 void setMemoryPermission(uptr Addr, uptr Size, uptr Flags,
                          UNUSED MapPlatformData *Data) {
   int Prot = (Flags & MAP_NOACCESS) ? PROT_NONE : (PROT_READ | PROT_WRITE);
   if (mprotect(reinterpret_cast<void *>(Addr), Size, Prot) != 0)
     dieOnMapUnmapError();
 }
 
-static bool madviseNeedsMemset() {
-  uptr Size = getPageSizeCached();
-  char *P = (char *)mmap(0, Size, PROT_READ | PROT_WRITE,
-                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-  if (!P)
-    dieOnMapUnmapError(errno == ENOMEM);
-  *P = 1;
-  while (madvise(P, Size, MADV_DONTNEED) == -1 && errno == EAGAIN) {
-  }
-  bool R = (*P != 0);
-  if (munmap(P, Size) != 0)
-    dieOnMapUnmapError();
-  return R;
-}
-
-static bool madviseNeedsMemsetCached() {
-  static atomic_u8 Cache;
-  enum State : u8 { Unknown = 0, Yes = 1, No = 2 };
-  State NeedsMemset = static_cast<State>(atomic_load_relaxed(&Cache));
-  if (NeedsMemset == Unknown) {
-    NeedsMemset = madviseNeedsMemset() ? Yes : No;
-    atomic_store_relaxed(&Cache, NeedsMemset);
-  }
-  return NeedsMemset == Yes;
-}
-
 void releasePagesToOS(uptr BaseAddress, uptr Offset, uptr Size, uptr Flags,
                       UNUSED MapPlatformData *Data) {
   void *Addr = reinterpret_cast<void *>(BaseAddress + Offset);
-  if (madviseNeedsMemsetCached()) {
-    // Workaround for QEMU-user ignoring MADV_DONTNEED.
-    // https://github.com/qemu/qemu/blob/b1cffefa1b163bce9aebc3416f562c1d3886eeaa/linux-user/syscall.c#L11941
-    // https://bugs.launchpad.net/qemu/+bug/1926521
-    int Prot = (Flags & MAP_NOACCESS) ? PROT_NONE : (PROT_READ | PROT_WRITE);
 #if defined(__aarch64__)
-    if (Flags & MAP_MEMTAG)
-      Prot |= PROT_MTE;
 #endif
-    void *P =
-        mmap(Addr, Size, Prot,
-             MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
-    if (P != Addr)
-      dieOnMapUnmapError();
-    return;
-  }
-  while (madvise(Addr, Size, MADV_DONTNEED) == -1 && errno == EAGAIN) {
-  }
 }
 
 // Calling getenv should be fine (c)(tm) at any time.
 const char *getEnv(const char *Name) { return getenv(Name); }
 
 namespace {
 enum State : u32 { Unlocked = 0, Locked = 1, Sleeping = 2 };
 }