Respect pgoff and rely on it for all types of binaries

This fixes incorrect attribution of events to symbols for ET_DYN objects (shared libraries and PIEs), and also simplifies the code.

Generally, the program linker assigns virtual addresses to segments, and virtual addresses of symbols stored in ELF are also in terms of that.
Upon mmapping a segment, its first byte conicides with the start of the memory mapping.
So to get the value of the program counter in terms of the DSO's virtual addresses, one has to add the virtual address of the segment to the raw PC's offset from the start of the mapping.
Like it is done in perf (see linux/tools/perf/util/map.h):
```
static inline u64 map__map_ip(struct map *map, u64 ip)
{
        return ip - map->start + map->pgoff;
}
```

(The addresses are translated in thread__find_map() via invoking the map_ip callback that has a comment saying /* ip -> dso rip */.)

OS Laboratory. Huawei Russian Research Institute. Saint-Petersburg

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D113648

Author: Pavel Kosov

lnt/testing/profile/cPerf.cpp

@@ -132,7 +132,7 @@ void Assert(bool Expr, const char *ExprStr, const char *File, int Line) {
 
 // Returns true if the ELF file given by filename
 // is a shared object (DYN).
-bool IsSharedObject(std::string Fname) {
+bool IsSharedObject(char *Fname) {
   // We replicate the first part of an ELF header here
   // so as not to rely on <elf.h>.
   struct PartialElfHeader {
@@ -141,7 +141,7 @@ bool IsSharedObject(std::string Fname) {
   };
   const int ET_DYN = 3;
 
-  FILE *stream = fopen(Fname.c_str(), "r");
+  FILE *stream = fopen(Fname, "r");
   if (stream == NULL)
     return false;
 
@@ -213,6 +213,8 @@ struct perf_event_mmap {
   char filename[1];
 };
 
+#define PROT_EXEC 4
+
 struct perf_event_mmap2 {
   struct perf_event_header header;
 
@@ -240,7 +242,7 @@ struct perf_sample_id {
 //===----------------------------------------------------------------------===//
 
 struct Map {
-  uint64_t Start, End;
+  uint64_t Start, End, Adjust;
   const char *Filename;
 };
 
@@ -544,7 +546,12 @@ unsigned char *PerfReader::readEvent(unsigned char *Buf) {
   if (E->header.type == PERF_RECORD_MMAP) {
     perf_event_mmap *E = (perf_event_mmap *)Buf;
     auto MapID = Maps.size();
-    Maps.push_back({E->start, E->start + E->extent, E->filename});
+    // EXEC ELF objects aren't relocated. DYN ones are,
+    // so if it's a DYN object adjust by subtracting the
+    // map base.
+    bool IsSO = IsSharedObject(E->filename);
+    Maps.push_back({E->start, E->start + E->extent,
+                    IsSO ? E->start - E->pgoff : 0, E->filename});
 
     // FIXME: use EventLayouts.begin()->second!
     perf_sample_id *ID =
@@ -554,8 +561,15 @@ unsigned char *PerfReader::readEvent(unsigned char *Buf) {
   }
   if (E->header.type == PERF_RECORD_MMAP2) {
     perf_event_mmap2 *E = (perf_event_mmap2 *)Buf;
+    if (!(E->prot & PROT_EXEC))
+      break;
     auto MapID = Maps.size();
-    Maps.push_back({E->start, E->start + E->extent, E->filename});
+    // EXEC ELF objects aren't relocated. DYN ones are,
+    // so if it's a DYN object adjust by subtracting the
+    // map base.
+    bool IsSO = IsSharedObject(E->filename);
+    Maps.push_back({E->start, E->start + E->extent,
+                    IsSO ? E->start - E->pgoff : 0, E->filename});
 
     // FIXME: use EventLayouts.begin()->second!
     perf_sample_id *ID =
@@ -705,11 +719,7 @@ void PerfReader::emitMaps() {
     if (AllUnderThreshold)
       continue;
 
-    // EXEC ELF objects aren't relocated. DYN ones are,
-    // so if it's a DYN object adjust by subtracting the
-    // map base.
-    bool IsSO = IsSharedObject(Maps[MapID].Filename);
-    uint64_t Adjust = IsSO ? Maps[MapID].Start : 0;
+    uint64_t Adjust = Maps[MapID].Adjust;
 
     NmOutput Syms(Nm, BinaryCacheRoot);
     Syms.reset(&Maps[MapID]);