[Support] Add error handling to sys::Process::getPageSize().

This patch changes the return type of sys::Process::getPageSize to
Expected<unsigned> to account for the fact that the underlying syscalls used to
obtain the page size may fail (see below).

For clients who use the page size as an optimization only this patch adds a new
method, getPageSizeEstimate, which calls through to getPageSize but discards
any error returned and substitues a "reasonable" page size estimate estimate
instead. All existing LLVM clients are updated to call getPageSizeEstimate
rather than getPageSize.

On Unix, sys::Process::getPageSize is implemented in terms of getpagesize or
sysconf, depending on which macros are set. The sysconf call is documented to
return -1 on failure. On Darwin getpagesize is implemented in terms of sysconf
and may also fail (though the manpage documentation does not mention this).
These failures have been observed in practice when highly restrictive sandbox
permissions have been applied. Without this patch, the result is that
getPageSize returns -1, which wreaks havoc on any subsequent code that was
assuming a sane page size value.

<rdar://problem/41654857>

Reviewers: dblaikie, echristo

Subscribers: kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59107

llvm-svn: 360221

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/Support/Unix/Memory.inc b/llvm/lib/Support/Unix/Memory.inc
index b8f5df5f72e..affdb7e70d9 100644
--- a/llvm/lib/Support/Unix/Memory.inc
+++ b/llvm/lib/Support/Unix/Memory.inc
@@ -103,7 +103,7 @@ Memory::allocateMappedMemory(size_t NumBytes,
   // Use any near hint and the page size to set a page-aligned starting address
   uintptr_t Start = NearBlock ? reinterpret_cast<uintptr_t>(NearBlock->base()) +
                                       NearBlock->size() : 0;
-  static const size_t PageSize = Process::getPageSize();
+  static const size_t PageSize = Process::getPageSizeEstimate();
   if (Start && Start % PageSize)
     Start += PageSize - Start % PageSize;
 
@@ -149,7 +149,7 @@ Memory::releaseMappedMemory(MemoryBlock &M) {
 
 std::error_code
 Memory::protectMappedMemory(const MemoryBlock &M, unsigned Flags) {
-  static const size_t PageSize = Process::getPageSize();
+  static const size_t PageSize = Process::getPageSizeEstimate();
   if (M.Address == nullptr || M.Size == 0)
     return std::error_code();