folly::ElfFile - add support to iterate over program headers

Summary: Adding iterateProgramHeaders, similar to iterateSections, just iterates over program headers instead of section headers. I want this to get the size of the first PT_LOAD header.

Differential Revision: D5602575

fbshipit-source-id: f73989cade20214f884571c1099761ecaa4841f7

folly/experimental/symbolizer/Elf-inl.h

@@ -21,6 +21,18 @@
 namespace folly {
 namespace symbolizer {
 
+template <class Fn>
+const ElfPhdr* ElfFile::iterateProgramHeaders(Fn fn) const {
+  const ElfPhdr* ptr = &at<ElfPhdr>(elfHeader().e_phoff);
+  for (size_t i = 0; i < elfHeader().e_phnum; i++, ptr++) {
+    if (fn(*ptr)) {
+      return ptr;
+    }
+  }
+
+  return nullptr;
+}
+
 template <class Fn>
 const ElfShdr* ElfFile::iterateSections(Fn fn) const {
   const ElfShdr* ptr = &at<ElfShdr>(elfHeader().e_shoff);

folly/experimental/symbolizer/Elf.cpp

@@ -263,24 +263,18 @@ bool ElfFile::init(const char** msg) {
     }
   }
 
-  const ElfPhdr* programHeader = &at<ElfPhdr>(elfHeader.e_phoff);
-  bool foundBase = false;
-  for (size_t i = 0; i < elfHeader.e_phnum; programHeader++, i++) {
   // Program headers are sorted by load address, so the first PT_LOAD
   // header gives us the base address.
-    if (programHeader->p_type == PT_LOAD) {
-      baseAddress_ = programHeader->p_vaddr;
-      foundBase = true;
-      break;
-    }
-  }
+  const ElfPhdr* programHeader =
+      iterateProgramHeaders([](auto& h) { return h.p_type == PT_LOAD; });
 
-  if (!foundBase) {
+  if (!programHeader) {
     if (msg) {
       *msg = "could not find base address";
     }
     return false;
   }
+  baseAddress_ = programHeader->p_vaddr;
 
   return true;
 }

folly/experimental/symbolizer/Elf.h

@@ -122,6 +122,14 @@ class ElfFile {
   template <class Fn>
   const char* iterateStrings(const ElfShdr& stringTable, Fn fn) const;
 
+  /**
+   * Iterate over program headers as long as fn(section) returns false.
+   * Returns a pointer to the current ("found") section when fn returned
+   * true, or nullptr if fn returned false for all sections.
+   */
+  template <class Fn>
+  const ElfPhdr* iterateProgramHeaders(Fn fn) const;
+
   /**
    * Iterate over all sections for as long as fn(section) returns false.
    * Returns a pointer to the current ("found") section when fn returned

folly/experimental/symbolizer/test/ElfTests.cpp

@@ -52,6 +52,13 @@ TEST_F(ElfTest, PointerValue) {
   EXPECT_STREQ(kStringValue, str);
 }
 
+TEST_F(ElfTest, iterateProgramHeaders) {
+  auto phdr = elfFile_.iterateProgramHeaders(
+      [](auto& h) { return h.p_type == PT_LOAD; });
+  EXPECT_NE(nullptr, phdr);
+  EXPECT_GE(phdr->p_filesz, 0);
+}
+
 int main(int argc, char** argv) {
   testing::InitGoogleTest(&argc, argv);
   gflags::ParseCommandLineFlags(&argc, &argv, true);