Add detailed message for IO_STRICT_DEVMEM kernel config

Also add this to the README.

Signed-off-by: Christian Blichmann <[email protected]>

README.md

@@ -26,8 +26,12 @@ The following command will extract the BIOS firmware and save the image to
 `bios_image.bin`:
 ```bash
 sudo .build/pawn bios_image.bin
-``` 
+```
 
-You can then use other tools like
+Note: When running a Linux kernel > 4.8.4, make sure that either
+`CONFIG_IO_DEVMEM=n` is set or that you've booted with the `iomem=relaxed`
+boot option.
+
+After extraction, you can then use other tools like
 [UEFITool](https://github.com/LongSoft/UEFITool) to process the firmware
 image further.

pawn.cc

@@ -62,10 +62,20 @@ int PawnMain(const char* dump_filename) {
       "Mapping 16KiB chipset configuration space at RCBA = 0x%8X, this may "
       "fail...\n",
       rcba.base_address);
-  QCHECK_OK(chipset->MapRootComplex(rcba));
+  status = chipset->MapRootComplex(rcba);
+  if (!status.ok()) {
+    printf(
+        "Error: %s\n"
+        "       Check if your kernel was compiled with IO_STRICT_DEVMEM=y.\n"
+        "       On Debian kernels > 4.8.4, boot with iomem=relaxed to\n"
+        "       temporarily disable /dev/mem IO protection.\n",
+        status.error_message().c_str());
+    return EXIT_FAILURE;
+  }
 
   auto gcs = chipset->ReadGcsRegister();
-  const char* kBootBiosStrapsDesc[] = {"LPC", "Reserved", "PCI", "SPI"};
+  constexpr const char* kBootBiosStrapsDesc[] = {"LPC", "Reserved", "PCI",
+                                                 "SPI"};
   printf("Boot BIOS Straps (BBS): %s\n",
          kBootBiosStrapsDesc[gcs.boot_bios_straps]);
   if (gcs.boot_bios_straps != Chipset::kBbsSpi) {

physical_memory.cc

@@ -51,6 +51,7 @@ util::Status PhysicalMemory::Init(uintptr_t physical_offset, size_t length) {
   mem_ = mmap(nullptr /* Address hint */, length_, PROT_READ | PROT_WRITE,
               MAP_SHARED, mem_fd_, physical_offset);
   if (mem_ == MAP_FAILED) {
+    mem_ = nullptr;
     string error(std::strerror(errno));
     return util::Status(util::error::FAILED_PRECONDITION,
                         string("Could not map physical memory: ") + error);