libcextract: Add support for Weak symbols for kernel mode

Until this point all symbols found in kernel mode (usually using Symvers file), were strong (with externalized variables) or none (which copied the symbols to the closure). Now clang-extract checks is the symbol is present on vmlinux, meaning that the symbol is weak externalized, meaning the the symbol will be always present. Signed-off-by: Marcos Paulo de Souza <[email protected]>
SUSE · Aug 15, 2024 · 55375f4 · 55375f4
1 parent 272ac2e
commit 55375f4
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Showing 5 changed files with 45 additions and 3 deletions.
diff --git a/libcextract/InlineAnalysis.cpp b/libcextract/InlineAnalysis.cpp
@@ -296,6 +296,12 @@ ExternalizationType InlineAnalysis::Needs_Externalization(const std::string &sym
 {
   if (Symv) {
     const std::string &sym_mod = Symv->Get_Symbol_Module(sym);
+    /* If the symbol comes from vmlinux, then we should use Weak
+     * externalization, since the symbol is always present when loading the
+     * livepatch. */
+    if (sym_mod == "vmlinux")
+      return ExternalizationType::WEAK;
+
     /*
      * If the symbol exists on Symvers we can decide whether the symbol must be
      * externalized or not, and not rely on ELF.

diff --git a/libcextract/Passes.cpp b/libcextract/Passes.cpp
@@ -538,9 +538,23 @@ class IbtTailGeneratePass : public Pass
     PassName = "IbtTailGeneratePass";
   }
 
+  /* There are a few cases where we don't want IBT tail declarations to be
+   * defined:
+   * * If IBT is not enabled
+   * * If there are not externalized symbols
+   * * If all the externalized symbols are weak
+   */
   virtual bool Gate(PassManager::Context *ctx)
   {
-    return ctx->Ibt && ctx->Externalize.size() > 0;
+    if (!ctx->Ibt || ctx->Externalize.size() == 0)
+      return false;
+
+    for (const ExternalizerLogEntry &entry : ctx->NamesLog) {
+      if (entry.Type == ExternalizationType::STRONG)
+        return true;
+    }
+
+    return false;
   }
 
   virtual bool Run_Pass(PassManager::Context *ctx)

diff --git a/libcextract/SymbolExternalizer.hh b/libcextract/SymbolExternalizer.hh
@@ -101,8 +101,13 @@ struct SymbolUpdateStatus
   }
 
   /* For IBT, NewDecl and OldDecl are the same if the symbols is a function, so
-     don't replace text if the name didn't change. */
+     don't replace text if the name didn't change.
+     In the case of WEAK externalization NewDecl will also be empty, but in this
+     case we don't want a symbol rename. */
   inline bool Needs_Sym_Rename(void) {
+    if (ExtType == ExternalizationType::WEAK)
+      return false;
+
     if (NewDecl == nullptr)
       return true;
 

diff --git a/testsuite/linux/Modules.symvers b/testsuite/linux/Modules.symvers
@@ -1 +1,2 @@
-0x00000000	crc32c	lib/libcrc32c	EXPORT_SYMBOL	
+0x00000000	crc32c	lib/libcrc32c	EXPORT_SYMBOL
+0x00000000	bpf_prog_put	vmlinux	EXPORT_SYMBOL_GPL
diff --git a/testsuite/linux/weak-symbols.c b/testsuite/linux/weak-symbols.c
@@ -0,0 +1,16 @@
+/* { dg-options "-DCE_EXTRACT_FUNCTIONS=f -DCE_SYMVERS_PATH=../testsuite/linux/Modules.symvers -DCE_RENAME_SYMBOLS -nostdinc -I../testsuite/linux -D__USE_IBT__ -D__KERNEL__" } */
+
+int bpf_prog_put(void);
+
+int bpf_prog_put(void)
+{
+	return 42;
+}
+
+int f(void)
+{
+	return bpf_prog_put();
+}
+
+/* { dg-final { scan-tree-dump "int bpf_prog_put\(void\);" } } */
+/* { dg-final { scan-tree-dump-not "return 42;" } } */