Re: [PATCH v19 00/10] Compile-time stack metadata validation

[Josh Poimboeuf]

On Tue, Mar 08, 2016 at 02:44:57PM +0100, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> > On Tue, Mar 08, 2016 at 11:37:16AM +0100, Ingo Molnar wrote:
> > > 
> > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > > 
> > > > This is v19 of the compile-time stack metadata validation patch set.
> > > > 
> > > > It's based on tip:core/objtool.
> > > 
> > > So I've upgraded my main build box to Fedora 23, to:
> > > 
> > >   gcc version 5.3.1 20151207 (Red Hat 5.3.1-2) (GCC)
> > >   GNU assembler version 2.25 (x86_64-redhat-linux) using BFD version version 2.25-15.fc23
> > >   GNU ld version 2.25-15.fc23
> > > 
> > > and I'm now getting a bunch of these warnings:
> > > 
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2e7: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2f3: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2ff: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x30b: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x317: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x323: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x32f: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x33b: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x348: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x355: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x362: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x36f: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x37c: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x389: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x396: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2e7: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2f3: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2ff: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x30b: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x317: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x323: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x32f: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x33b: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x348: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x355: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x362: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x36f: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x37c: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x389: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x396: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x19a: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1a6: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1b2: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1be: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1ca: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1d6: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1e2: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1ee: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x1ff: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x210: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x221: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x251: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x262: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x273: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x284: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x295: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2a6: frame pointer state mismatch
> > > drivers/net/wireless/realtek/rtlwifi/base.o: warning: objtool: rtlwifi_rate_mapping()+0x2b7: frame pointer state mismatch
> > > 
> > > I see nothing particularly weird about rtlwifi_rate_mapping(), except the tons of 
> > > switch statements - maybe GCC does something really clever here?
> > > 
> > > In any case this needs to be addressed before the merge window. Also, I think we 
> > > should limit the number of objtool warnings to one per function?
> > 
> > This is a bug in objtool's handling of switch statement jump tables.  In
> > fact, the 0-day bot warned me about this bug yesterday, but I
> > misinterpreted it:
> > 
> >   https://lkml.kernel.org/r/201603060006.yNZ9xTcs%fengguang.wu@xxxxxxxxx
> > 
> > I'll fix it up today.  I'll also limit the warnings to one per function.
> > 
> > BTW, I was only able to recreate this particular issue after updating to
> > today's tip/master.  It was triggered by a new config option:
> > CONFIG_UBSAN_SANITIZE_ALL.
> 
> ah, makes sense! I also upgraded to the latestest Fedora and misinterpreted the 
> warning.
> 
> Btw., I like it how in other cases you didn't try to work the warning around in 
> objtool, but improved the C code itself. We can consider this warning as a 
> 'unnecessarily complex switch statement' warning.

So the bug is that when there's more than one switch statement in a
function, objtool interprets all the switch jump tables as a single
table.  If the targets of one jump table assume a stack frame and the
targets of another one don't, you'll see frame pointer warnings like
those above.

It *is* interesting that both functions which triggered this bug --
rtlwifi_rate_mapping() and cik_tiling_mode_table_init() -- are cases
where the code is way too complex and can be simplified.

But, unless we want to institute an "only one switch statement allowed
per function" policy, I think I should still fix the bug.  Because I
don't really have the clout to tell people "you need to clean up your
code because my tool is buggy." :-)

-- 
Josh