Commit 08b393d0 authored Jun 27, 2018 by Josh Poimboeuf Committed by Ingo Molnar Jul 02, 2018

objtool: Support GCC 8 '-fnoreorder-functions'



Since the following commit:

  cd77849a ("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@oracle.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Allan Xavier <allan.x.xavier@oracle.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/9e704c52bee651129b036be14feda317ae5606ae.1530136978.git.jpoimboe@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 021c9179

tools/objtool/elf.c

+28 −13

Original line number	Diff line number	Diff line
		@@ -302,7 +302,9 @@ static int read_symbols(struct elf *elf)
		continue;
		sym->pfunc = sym->cfunc = sym;
		coldstr = strstr(sym->name, ".cold.");
		if (coldstr) {
		if (!coldstr)
		continue;

		coldstr[0] = '\0';
		pfunc = find_symbol_by_name(elf, sym->name);
		coldstr[0] = '.';
		@@ -315,6 +317,19 @@ static int read_symbols(struct elf *elf)

		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;
		}
		}
		}