linux-kernel - [RFC PATCH 2/2] pgo: Make module.lds.S linker script to merge __llvm_prf

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Message-Id: <20211123012138.160532-3-jarmo.tiitto@gmail.com>
Date:   Tue, 23 Nov 2021 03:21:37 +0200
From:   Jarmo Tiitto <jarmo.tiitto@...il.com>
To:     Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, Jessica Yu <jeyu@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Jarmo Tiitto <jarmo.tiitto@...il.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Cc:     Bill Wendling <wcw@...gle.com>, Joe Perches <joe@...ches.com>
Subject: [RFC PATCH 2/2] pgo: Make module.lds.S linker script to merge __llvm_prf_ sections.

The compiler leaves many duplicate instances of __llvm_prf_* sections
in final module objects and they confuse the kernel module loader.
Fix this by providing module.lds.S linker script that merges the split
sections like vmlinux.lds.S currently does.

Signed-off-by: Jarmo Tiitto <jarmo.tiitto@...il.com>
---
I'm not sure the linker script is entirely correct and
it propably needs more work.
---
 scripts/module.lds.S | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 04c5685c25cf..83ca5825c0c3 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -60,6 +60,39 @@ SECTIONS {
 		*(.text .text.[0-9a-zA-Z_]* .text..L.cfi*)
 	}
 #endif
+#ifdef CONFIG_PGO_CLANG
+	/*
+	 * With CONFIG_PGO_CLANG the compiler may split __llvm_prf_xxx
+	 * objects into multiple sections. Merge them in final .ko object.
+	 * However leave .rela__llvm_prf_data sections as-is
+	 * since they are needed by the module loader.
+	 */
+	__llvm_prf_data : AT(ADDR(__llvm_prf_data)) {
+		__llvm_prf_data_start = .;
+		KEEP(*(SORT(__llvm_prf_data)))
+		__llvm_prf_data_end = .;
+	}
+	__llvm_prf_cnts : AT(ADDR(__llvm_prf_cnts)) {
+		__llvm_prf_cnts_start = .;
+		KEEP(*(SORT(__llvm_prf_cnts)))
+		__llvm_prf_cnts_end = .;
+	}
+	__llvm_prf_names : AT(ADDR(__llvm_prf_names)) {
+		__llvm_prf_names_start = .;
+		KEEP(*(SORT(__llvm_prf_names)))
+		__llvm_prf_names_end = .;
+	}
+	__llvm_prf_vals : AT(ADDR(__llvm_prf_vals)) {
+		__llvm_prf_vals_start = .;
+		KEEP(*(SORT(__llvm_prf_vals)))
+		__llvm_prf_vals_end = .;
+	}
+	__llvm_prf_vnds : AT(ADDR(__llvm_prf_vnds)) {
+		__llvm_prf_vnds_start = .;
+		KEEP(*(SORT(__llvm_prf_vnds)))
+		__llvm_prf_vnds_end = .;
+	}
+#endif
 }
 
 /* bring in arch-specific sections */
-- 
2.34.0