Remove variable-length arrays in the macOS disk image sniffer

This enables compiling this code with -Wvla.

safe_browsing::DiskImageTypeSnifferMac::AppleDiskImageTrailerSize was a
non-constexpr function whose implementation was hidden in
disk_image_type_sniffer_mac.cc, preventing its return value, no matter
how constant it actually was, from being used to size arrays statically
at compile time in other translation units, such as
archive_analyzer_results.cc. This changes it to be a constexpr variable,
kAppleDiskImageTrailerSize, which can be used to size arrays statically
at compile time.

Change-Id: I35d4582ce8446c9f90d8a6d8576c9d60115c5de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363069Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799215}

chrome/common/safe_browsing/archive_analyzer_results.cc

@@ -113,9 +113,9 @@ void UpdateArchiveAnalyzerResultsWithFile(base::FilePath path,
   uint32_t magic;
   file->Read(0, reinterpret_cast<char*>(&magic), sizeof(uint32_t));
 
-  char dmg_header[DiskImageTypeSnifferMac::AppleDiskImageTrailerSize()];
+  char dmg_header[DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize];
   file->Read(0, dmg_header,
-             DiskImageTypeSnifferMac::AppleDiskImageTrailerSize());
+             DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize);
 
   current_entry_is_executable =
       FileTypePolicies::GetInstance()->IsCheckedBinaryFile(path) ||
@@ -123,20 +123,20 @@ void UpdateArchiveAnalyzerResultsWithFile(base::FilePath path,
       DiskImageTypeSnifferMac::IsAppleDiskImageTrailer(
           base::span<const uint8_t>(
               reinterpret_cast<const uint8_t*>(dmg_header),
-              DiskImageTypeSnifferMac::AppleDiskImageTrailerSize()));
+              DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize));
 
   // We can skip checking the trailer if we already know the file is executable.
   if (!current_entry_is_executable) {
-    char trailer[DiskImageTypeSnifferMac::AppleDiskImageTrailerSize()];
+    char trailer[DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize];
     file->Seek(base::File::Whence::FROM_END,
-               DiskImageTypeSnifferMac::AppleDiskImageTrailerSize());
-    file->ReadAtCurrentPos(
-        trailer, DiskImageTypeSnifferMac::AppleDiskImageTrailerSize());
+               DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize);
+    file->ReadAtCurrentPos(trailer,
+                           DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize);
     current_entry_is_executable =
         DiskImageTypeSnifferMac::IsAppleDiskImageTrailer(
             base::span<const uint8_t>(
                 reinterpret_cast<const uint8_t*>(trailer),
-                DiskImageTypeSnifferMac::AppleDiskImageTrailerSize()));
+                DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize));
   }
 
 #else

chrome/common/safe_browsing/disk_image_type_sniffer_mac.cc

@@ -11,7 +11,6 @@ namespace {
 
 const uint8_t kKolySignature[4] = {'k', 'o', 'l', 'y'};
 constexpr size_t kSizeKolySignatureInBytes = sizeof(kKolySignature);
-const size_t kSizeKolyTrailerInBytes = 512;
 
 }  // namespace
 
@@ -30,7 +29,7 @@ bool DiskImageTypeSnifferMac::IsAppleDiskImage(const base::FilePath& dmg_file) {
 
   char data[kSizeKolySignatureInBytes];
 
-  if (file.Seek(base::File::FROM_END, -1 * kSizeKolyTrailerInBytes) == -1)
+  if (file.Seek(base::File::FROM_END, -1 * kAppleDiskImageTrailerSize) == -1)
     return false;
 
   if (file.ReadAtCurrentPos(data, kSizeKolySignatureInBytes) !=
@@ -54,11 +53,8 @@ bool DiskImageTypeSnifferMac::IsAppleDiskImageTrailer(
           0);
 }
 
-// static
-size_t DiskImageTypeSnifferMac::AppleDiskImageTrailerSize() {
-  return kSizeKolyTrailerInBytes;
-}
-
 DiskImageTypeSnifferMac::~DiskImageTypeSnifferMac() = default;
 
+constexpr size_t DiskImageTypeSnifferMac::kAppleDiskImageTrailerSize;
+
 }  // namespace safe_browsing

chrome/common/safe_browsing/disk_image_type_sniffer_mac.h

@@ -27,8 +27,8 @@ class DiskImageTypeSnifferMac
   // Returns true when the trailer is a valid trailer for a DMG type.
   static bool IsAppleDiskImageTrailer(const base::span<const uint8_t>& trailer);
 
-  // Returns the size of a DMG trailer.
-  static size_t AppleDiskImageTrailerSize();
+  // The size of a DMG trailer.
+  static constexpr size_t kAppleDiskImageTrailerSize = 512;
 
  private:
   friend class base::RefCountedThreadSafe<DiskImageTypeSnifferMac>;