Detect kernel headers even if they are splitted into source/ and build/ directories

Currently bpftrace uses only /lib/modules/`uname -r`/source/ if it
exists. This however leads to failures Debian where generated kernel
headers, such as include/generated/autoconf.h are put into a separate
directory, for example:

	---- 8< ---- (fuse_vs_mmapsem.bt)
	#include <linux/path.h>
	#include <linux/dcache.h>
	#include <linux/sched.h>
	#include <linux/mm_types.h>

	kprobe:fuse_readpage,kprobe:fuse_readpages {
	        printf("%s (%d) %s\n", probe, ((task_struct *)curtask)->mm->mmap_sem.count.counter, kstack);
	}
	---- 8< ----

	# ./bpftrace fuse_vs_mmapsem.bt
	/lib/modules/4.19.0-2-amd64/source/include/linux/kconfig.h:5:10: fatal error: 'generated/autoconf.h' file not found
	Unknown struct/union: 'task_struct'

Fix it by using both /lib/modules/`uname -r`/source/ and
/lib/modules/`uname -r`/build/ if both are present.

/cc @vincentbernat

P.S. I was not using C++ for ages, so please forgive me in advance if I
missed something.

src/clang_parser.cpp

@@ -122,21 +122,54 @@ static bool is_dir(const std::string& path)
   return S_ISDIR(buf.st_mode);
 }
 
-static std::string get_kernel_root_dir(const struct utsname& utsname)
+// get_kernel_dirs returns {ksrc, kobj} - directories for pristine and
+// generated kernel sources.
+//
+// When the kernel was built in its source tree ksrc == kobj, however when
+// the kernel was build in a different directory than its source, ksrc != kobj.
+//
+// A notable example is Debian, which places pristine kernel headers in
+//
+//	/lib/modules/`uname -r`/source/
+//
+// and generated kernel headers in
+//
+//	/lib/modules/`uname -r`/build/
+//
+// {"", ""} is returned if no trace of kernel headers was found at all.
+// Both ksrc and kobj are guaranteed to be != "", if at least some trace of kernel sources was found.
+static std::tuple<std::string, std::string> get_kernel_dirs(const struct utsname& utsname)
 {
 #ifdef KERNEL_HEADERS_DIR
-  return KERNEL_HEADERS_DIR;
+  return {KERNEL_HEADERS_DIR, KERNEL_HEADERS_DIR};
 #endif
 
   const char *kpath_env = ::getenv("BPFTRACE_KERNEL_SOURCE");
   if (kpath_env)
-    return kpath_env;
+    return {kpath_env, kpath_env};
 
   std::string kdir = std::string("/lib/modules/") + utsname.release;
-  if (is_dir(kdir + "/build") && is_dir(kdir + "/source"))
-    return kdir + "/source";
-  else
-    return kdir + "/build";
+  auto ksrc = kdir + "/source";
+  auto kobj = kdir + "/build";
+
+  // if one of source/ or build/ is not present - try to use the other one for both.
+  if (!is_dir(ksrc)) {
+	  ksrc = "";
+  }
+  if (!is_dir(kobj)) {
+	  kobj = "";
+  }
+  if (ksrc == "" && kobj == "") {
+	  return {"", ""};
+  }
+  if (ksrc == "") {
+	  ksrc = kobj;
+  }
+  else if (kobj == "") {
+	  kobj = ksrc;
+  }
+
+  return {ksrc, kobj};
 }
 
 void ClangParser::parse(ast::Program *program, StructMap &structs)
@@ -187,9 +220,9 @@ void ClangParser::parse(ast::Program *program, StructMap &structs)
   std::vector<std::string> kflags;
   struct utsname utsname;
   uname(&utsname);
-  auto kpath = get_kernel_root_dir(utsname);
-  if (is_dir(kpath))
-    kflags = get_kernel_cflags(utsname.machine, kpath);
+  auto [ksrc, kobj] = get_kernel_dirs(utsname);
+  if (ksrc != "")
+    kflags = get_kernel_cflags(utsname.machine, ksrc, kobj);
 
   std::vector<const char *> args =
   {

src/utils.cpp

@@ -53,7 +53,8 @@ std::vector<int> get_possible_cpus()
 
 std::vector<std::string> get_kernel_cflags(
     const char* uname_machine,
-    const std::string& kdir)
+    const std::string& ksrc,
+    const std::string& kobj)
 {
   std::vector<std::string> cflags;
   std::string arch = uname_machine;
@@ -90,18 +91,18 @@ std::vector<std::string> get_kernel_cflags(
   cflags.push_back("-isystem");
   cflags.push_back("/virtual/lib/clang/include");
 
-  cflags.push_back("-I" + kdir + "/arch/"+arch+"/include");
-  cflags.push_back("-I" + kdir + "/arch/"+arch+"/include/generated/uapi");
-  cflags.push_back("-I" + kdir + "/arch/"+arch+"/include/generated");
-  cflags.push_back("-I" + kdir + "/include");
-  cflags.push_back("-I" + kdir + "/./arch/"+arch+"/include/uapi");
-  cflags.push_back("-I" + kdir + "/arch/"+arch+"/include/generated/uapi");
-  cflags.push_back("-I" + kdir + "/include/uapi");
-  cflags.push_back("-I" + kdir + "/include/generated");
-  cflags.push_back("-I" + kdir + "/include/generated/uapi");
+  // see linux/Makefile for $(LINUXINCLUDE) + $(USERINCLUDE)
+  cflags.push_back("-I" + ksrc + "/arch/"+arch+"/include");
+  cflags.push_back("-I" + kobj + "/arch/"+arch+"/include/generated");
+  cflags.push_back("-I" + ksrc + "/include");
+  cflags.push_back("-I" + kobj + "/include");
+  cflags.push_back("-I" + ksrc + "/arch/"+arch+"/include/uapi");
+  cflags.push_back("-I" + kobj + "/arch/"+arch+"/include/generated/uapi");
+  cflags.push_back("-I" + ksrc + "/include/uapi");
+  cflags.push_back("-I" + kobj + "/include/generated/uapi");
 
   cflags.push_back("-include");
-  cflags.push_back(kdir + "/include/linux/kconfig.h");
+  cflags.push_back(ksrc + "/include/linux/kconfig.h");
   cflags.push_back("-D__KERNEL__");
   cflags.push_back("-D__BPF_TRACING__");
   cflags.push_back("-D__HAVE_BUILTIN_BSWAP16__");

src/utils.h

@@ -26,7 +26,8 @@ std::vector<int> get_online_cpus();
 std::vector<int> get_possible_cpus();
 std::vector<std::string> get_kernel_cflags(
     const char* uname_machine,
-    const std::string& kdir);
+    const std::string& ksrc,
+    const std::string& kobj);
 std::string is_deprecated(std::string &str);
 
 } // namespace bpftrace