linux-kernel - [tip:core/urgent] objtool: Support GCC 8 '-fnoreorder-functions'

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Date:   Mon, 2 Jul 2018 00:46:02 -0700
From:   tip-bot for Josh Poimboeuf <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     dvlasenk@...hat.com, luto@...nel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, hpa@...or.com, jpoimboe@...hat.com,
        torvalds@...ux-foundation.org, bp@...en8.de,
        allan.x.xavier@...cle.com, tglx@...utronix.de, brgerst@...il.com,
        mingo@...nel.org
Subject: [tip:core/urgent] objtool: Support GCC 8 '-fnoreorder-functions'

Commit-ID:  08b393d01c88aff27347ed2b1b354eb4db2f1532
Gitweb:     https://git.kernel.org/tip/08b393d01c88aff27347ed2b1b354eb4db2f1532
Author:     Josh Poimboeuf <jpoimboe@...hat.com>
AuthorDate: Wed, 27 Jun 2018 17:03:45 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 2 Jul 2018 09:42:11 +0200

objtool: Support GCC 8 '-fnoreorder-functions'

Since the following commit:

  cd77849a69cf ("objtool: Fix GCC 8 cold subfunction detection for aliased functions")

... if the kernel is built with EXTRA_CFLAGS='-fno-reorder-functions',
objtool can get stuck in an infinite loop.

That flag causes the new GCC 8 cold subfunctions to be placed in .text
instead of .text.unlikely.  But it also has an unfortunate quirk: in the
symbol table, the subfunction (e.g., nmi_panic.cold.7) is nested inside
the parent (nmi_panic).

That function overlap confuses objtool, and causes it to get into an
infinite loop in next_insn_same_func().  Here's Allan's description of
the loop:

  "Objtool iterates through the instructions in nmi_panic using
  next_insn_same_func. Once it reaches the end of nmi_panic at 0x534 it
  jumps to 0x528 as that's the start of nmi_panic.cold.7. However, since
  the instructions starting at 0x528 are still associated with nmi_panic
  objtool will get stuck in a loop, continually jumping back to 0x528
  after reaching 0x534."

Fix it by shortening the length of the parent function so that the
functions no longer overlap.

Reported-and-analyzed-by: Allan Xavier <allan.x.xavier@...cle.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Allan Xavier <allan.x.xavier@...cle.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/9e704c52bee651129b036be14feda317ae5606ae.1530136978.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 tools/objtool/elf.c | 41 ++++++++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 4e60e105583e..0d1acb704f64 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -302,19 +302,34 @@ static int read_symbols(struct elf *elf)
 				continue;
 			sym->pfunc = sym->cfunc = sym;
 			coldstr = strstr(sym->name, ".cold.");
-			if (coldstr) {
-				coldstr[0] = '\0';
-				pfunc = find_symbol_by_name(elf, sym->name);
-				coldstr[0] = '.';
-
-				if (!pfunc) {
-					WARN("%s(): can't find parent function",
-					     sym->name);
-					goto err;
-				}
-
-				sym->pfunc = pfunc;
-				pfunc->cfunc = sym;
+			if (!coldstr)
+				continue;
+
+			coldstr[0] = '\0';
+			pfunc = find_symbol_by_name(elf, sym->name);
+			coldstr[0] = '.';
+
+			if (!pfunc) {
+				WARN("%s(): can't find parent function",
+				     sym->name);
+				goto err;
+			}
+
+			sym->pfunc = pfunc;
+			pfunc->cfunc = sym;
+
+			/*
+			 * Unfortunately, -fnoreorder-functions puts the child
+			 * inside the parent.  Remove the overlap so we can
+			 * have sane assumptions.
+			 *
+			 * Note that pfunc->len now no longer matches
+			 * pfunc->sym.st_size.
+			 */
+			if (sym->sec == pfunc->sec &&
+			    sym->offset >= pfunc->offset &&
+			    sym->offset + sym->len == pfunc->offset + pfunc->len) {
+				pfunc->len -= sym->len;
 			}
 		}
 	}