linux-kernel - Re: [PATCH] objtool: Fix GCC 8 cold function processing without -freorder-functions

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Date:   Tue, 26 Jun 2018 13:44:05 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Allan Xavier <allan.x.xavier@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: Fix GCC 8 cold function processing without
 -freorder-functions

On Tue, Jun 26, 2018 at 07:31:18PM +0100, Allan Xavier wrote:
> Hi Peter,
> 
> On 26/06/18 17:43, Peter Zijlstra wrote:
> > On Tue, Jun 26, 2018 at 05:20:45PM +0100, Allan Xavier wrote:
> >> 0000000000000500 g     F .text  0000000000000034 nmi_panic
> >> 0000000000000528 l     F .text  000000000000000c nmi_panic.cold.7
> >>
> >> This doesn't happen with -freorder-functions in the first example as the
> >> symbols don't overlap. 
> > 
> > Urgh and I don't suppose we can 'fix' the overlap in read_symbols() ?
> > 
> 
> It should be fixable in read_symbols too if you don't mind sym->len not being
> the same as sym->st_size in the ELF.
> 
> Is there a particular concern you're trying to address by having the logic there
> instead?
> 
> Will have a look into reworking the patch in any case.

Thanks for the patch and the excellent problem description.  I think the
concern is that the overlap might cause other unforeseen issues (now or
in the future).  How about something like this?  (The diff also includes
a cleanup to reduce the indent level.)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 4e60e105583e..36518393a960 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -302,19 +302,32 @@ 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.
+			 */
+			if (sym->sec == pfunc->sec &&
+			    sym->offset >= pfunc->offset &&
+			    sym->offset < pfunc->offset + pfunc->len &&
+			    sym->offset + sym->len == pfunc->offset + pfunc->len) {
+				pfunc->len -= sym->len;
 			}
 		}
 	}