From 131484c8da97ed600c18dd9d03b661e8ae052df6 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Thu, 28 May 2015 12:21:47 +0200 Subject: x86/debug: Remove perpetually broken, unmaintainable dwarf annotations MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit So the dwarf2 annotations in low level assembly code have become an increasing hindrance: unreadable, messy macros mixed into some of the most security sensitive code paths of the Linux kernel. These debug info annotations don't even buy the upstream kernel anything: dwarf driven stack unwinding has caused problems in the past so it's out of tree, and the upstream kernel only uses the much more robust framepointers based stack unwinding method. In addition to that there's a steady, slow bitrot going on with these annotations, requiring frequent fixups. There's no tooling and no functionality upstream that keeps it correct. So burn down the sick forest, allowing new, healthier growth: 27 files changed, 350 insertions(+), 1101 deletions(-) Someone who has the willingness and time to do this properly can attempt to reintroduce dwarf debuginfo in x86 assembly code plus dwarf unwinding from first principles, with the following conditions: - it should be maximally readable, and maximally low-key to 'ordinary' code reading and maintenance. - find a build time method to insert dwarf annotations automatically in the most common cases, for pop/push instructions that manipulate the stack pointer. This could be done for example via a preprocessing step that just looks for common patterns - plus special annotations for the few cases where we want to depart from the default. We have hundreds of CFI annotations, so automating most of that makes sense. - it should come with build tooling checks that ensure that CFI annotations are sensible. We've seen such efforts from the framepointer side, and there's no reason it couldn't be done on the dwarf side. Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: Frédéric Weisbecker Cc: Jan Beulich Cc: Josh Poimboeuf Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Signed-off-by: Ingo Molnar --- arch/x86/lib/clear_page_64.S | 7 ------- 1 file changed, 7 deletions(-) (limited to 'arch/x86/lib/clear_page_64.S') diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S index e67e579c93bd..a2fe51b00cce 100644 --- a/arch/x86/lib/clear_page_64.S +++ b/arch/x86/lib/clear_page_64.S @@ -1,5 +1,4 @@ #include -#include #include #include @@ -15,7 +14,6 @@ * %rdi - page */ ENTRY(clear_page) - CFI_STARTPROC ALTERNATIVE_2 "jmp clear_page_orig", "", X86_FEATURE_REP_GOOD, \ "jmp clear_page_c_e", X86_FEATURE_ERMS @@ -24,11 +22,9 @@ ENTRY(clear_page) xorl %eax,%eax rep stosq ret - CFI_ENDPROC ENDPROC(clear_page) ENTRY(clear_page_orig) - CFI_STARTPROC xorl %eax,%eax movl $4096/64,%ecx @@ -48,14 +44,11 @@ ENTRY(clear_page_orig) jnz .Lloop nop ret - CFI_ENDPROC ENDPROC(clear_page_orig) ENTRY(clear_page_c_e) - CFI_STARTPROC movl $4096,%ecx xorl %eax,%eax rep stosb ret - CFI_ENDPROC ENDPROC(clear_page_c_e) -- cgit v1.2.3