[BOLT] Detect Linux kernel version if the binary is a Linux kernel

This makes it easier to handle differences (e.g. of exception table
entry size) between versions of Linux kernel

Author: FLZ101

bolt/include/bolt/Core/BinaryContext.h

@@ -670,6 +670,9 @@ class BinaryContext {
   /// Indicates if the binary is Linux kernel.
   bool IsLinuxKernel{false};
 
+  /// Linux kernel version (major, minor, reversion)
+  std::tuple<unsigned, unsigned, unsigned> LinuxKernelVersion;
+
   /// Indicates if relocations are available for usage.
   bool HasRelocations{false};
 

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -61,6 +61,7 @@
 #include <fstream>
 #include <memory>
 #include <optional>
+#include <regex>
 #include <system_error>
 
 #undef  DEBUG_TYPE
@@ -1030,6 +1031,25 @@ void RewriteInstance::discoverFileObjects() {
       continue;
     }
 
+    if (BC->IsLinuxKernel && SymName == "linux_banner") {
+      const StringRef SectionContents =
+          cantFail(Section->getContents(), "can not get section contents");
+      const std::string S =
+          SectionContents
+              .substr(SymbolAddress - Section->getAddress(), SymbolSize)
+              .str();
+
+      const std::regex Re(R"---(Linux version ((\d+)\.(\d+)(\.(\d+))?))---");
+      std::smatch Match;
+      if (std::regex_search(S, Match, Re)) {
+        unsigned Major = std::stoi(Match[2].str());
+        unsigned Minor = std::stoi(Match[3].str());
+        unsigned Rev = Match.size() > 5 ? std::stoi(Match[5].str()) : 0;
+        BC->LinuxKernelVersion = std::make_tuple(Major, Minor, Rev);
+        BC->outs() << "BOLT-INFO: Linux kernel version is " << Match[1].str();
+      }
+    }
+
     if (!Section->isText()) {
       assert(SymbolType != SymbolRef::ST_Function &&
              "unexpected function inside non-code section");
@@ -1205,6 +1225,9 @@ void RewriteInstance::discoverFileObjects() {
     PreviousFunction = BF;
   }
 
+  if (BC->IsLinuxKernel && !std::get<0>(BC->LinuxKernelVersion))
+    BC->errs() << "BOLT-WARNING: Linux kernel version is unknown\n";
+
   // Read dynamic relocation first as their presence affects the way we process
   // static relocations. E.g. we will ignore a static relocation at an address
   // that is a subject to dynamic relocation processing.

bolt/test/X86/linux-version.s

@@ -0,0 +1,30 @@
+# REQUIRES: system-linux
+
+## Check that BOLT correctly detects the Linux kernel version
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %s -o %t.o
+# RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
+# RUN:   -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr
+
+# RUN: llvm-bolt %t.exe -o %t.out | FileCheck %s
+
+# CHECK: BOLT-INFO: Linux kernel version is 6.6.61
+
+	.text
+	.globl	f
+	.type	f, @function
+f:
+	ret
+	.size	f, .-f
+
+	.globl	linux_banner
+	.section	.rodata
+	.align 16
+	.type	linux_banner, @object
+	.size	linux_banner, 22
+linux_banner:
+	.string	"Linux version 6.6.61\n"
+
+## Fake Linux Kernel sections.
+  .section __ksymtab,"a",@progbits
+  .section __ksymtab_gpl,"a",@progbits