[Zucchini] Rel32Finder: Make rel32 accept / reject semantics explicit.

Zucchini heuristically scans assembly code byte-by-byte for rel32
references. When found, the result needs validation, and on X86 / X64,
this directs where to scan next: If accepted, scan resumes after the
instruction containing the rel32 found; if rejected, scan resumes on
the next byte.

Rel32Finder implements the above interactively: GetNext() emits the
next candidate rel32, and the caller needs to call Accept() to signal
acceptance (else rejection is assumed).

Inherited classes of Rel32Finder implements architecture-specific code
via Scan(), which caches results. Previously, Scan() also returns a
range for the instruction found. If accepted, scan resumes after the
range; if rejected, scan resumes 1 byte after the start of range.

Problem: The "scan 1 byte after" scheme works well for X86 / X64 and
fixed-size instructions (by aligning in Scan()). However, for THUMB2
instructions in ARM, which has easily discernible 2-byte and 4-byte
op codes, for both "accept" and "reject", scan should resume on the
next instruction.

This CL refactors Rel32Finder to solve the above, with other cleanup.
Details:
* Change Scan() to return (new struct) NextIterators, which stores
  iterator for "accept" and "reject" cases.
* Rename Reset() to SetRegion() to assign |region_|, and remove the
  |region_| assignment via constructor.
* Add Rel32FinderIntel::SetResult().
* Move more code from .h to .cc.
* Rename |next_cursor_| to |accept_it_|.
* Extensive comment updates.

Bug: 943315,918867
Change-Id: Ie0a0b380975c35b0aedb013037f8d69673c9697c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529166
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643098}
NOKEYCHECK=True
GitOrigin-RevId: 47fef62aa7626d9a47fc1986b8e51e6e866570d1

1 files changed, 5 insertions, 6 deletions

diff --git a/disassembler_elf.cc b/disassembler_elf.cc
index ef050a4..474f43e 100644
--- a/disassembler_elf.cc
+++ b/disassembler_elf.cc
@@ -371,13 +371,12 @@ void DisassemblerElfIntel<Traits>::ParseExecSection(
 
   ConstBufferView region(image_.begin() + section.sh_offset, section.sh_size);
   Abs32GapFinder gap_finder(image_, region, abs32_locations_, 4);
-  std::unique_ptr<Rel32FinderIntel> finder =
-      std::make_unique<typename Traits::Rel32FinderUse>(image_);
+  typename Traits::Rel32FinderUse finder;
   for (auto gap = gap_finder.GetNext(); gap.has_value();
        gap = gap_finder.GetNext()) {
-    finder->Reset(gap.value());
-    for (auto rel32 = finder->GetNext(); rel32.has_value();
-         rel32 = finder->GetNext()) {
+    finder.SetRegion(gap.value());
+    for (auto rel32 = finder.GetNext(); rel32.has_value();
+         rel32 = finder.GetNext()) {
       offset_t rel32_offset =
           base::checked_cast<offset_t>(rel32->location - image_.begin());
       rva_t rel32_rva = rva_t(rel32_offset + from_offset_to_rva);
@@ -385,7 +384,7 @@ void DisassemblerElfIntel<Traits>::ParseExecSection(
       if (target_rva_checker.IsValid(target_rva) &&
           (rel32->can_point_outside_section ||
            (start_rva <= target_rva && target_rva < end_rva))) {
-        finder->Accept();
+        finder.Accept();
         rel32_locations_.push_back(rel32_offset);
       }
     }