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Message-ID: <YYENAKB0igNFnFmK@hirez.programming.kicks-ass.net> Date: Tue, 2 Nov 2021 11:03:44 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Josh Poimboeuf <jpoimboe@...hat.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, the arch/x86 maintainers <x86@...nel.org>, Alexander Popov <alex.popov@...ux.com> Subject: Re: Stackleak vs noinstr (Was: [GIT pull] objtool/core for v5.16-rc1) On Tue, Nov 02, 2021 at 10:05:50AM +0100, Peter Zijlstra wrote: > On Tue, Nov 02, 2021 at 09:00:36AM +0100, Peter Zijlstra wrote: > > On Mon, Nov 01, 2021 at 01:44:39PM -0700, Linus Torvalds wrote: > > > > do_machine_check()+0x27: call to stackleak_track_stack ... > > > do_syscall_64()+0x9: call to stackleak_track_stack ... > > > do_int80_syscall_32()+0x9: call to stackleak_track_stack ... > > > exc_general_protection()+0x22: call to stackleak_track_stack ... > > > fixup_bad_iret()+0x20: call to stackleak_track_stack ... > > > .entry.text+0x10e6: call to stackleak_erase ... > > > .entry.text+0x143: call to stackleak_erase ... > > > .entry.text+0x17d9: call to stackleak_erase ... > > > > > > most seem to be about the stackleak thing, > > > > Right, I recently ran into this and hacen't yet had time to look into > > it. I suspect my normal build box doesn't have the GCC plugin crud > > enabled or somesuch. > > > > I think the GCC stackleak plugin needs fixing, specifically it needs a > > function attribute such that it will not emit instrumentation in noinstr > > functions. I'll go chase down the developer of that thing. > > Alexander, is there any way to make this plugin grow a function > attribute which we can add to noinstr ? There's a strict requirement the > compiler doesn't add extra code to noinstr functions these days. > > We'll 'soon' be running noinstr C code before switching to kernel page > tables even. Using my pre-release GCC-12 compiler (the only one I have with plugin crud enabled apparently), the below seems to work. Having the plugin gate on section name seems a lot hacky, but given it's already doing that, one more doesn't hurt. --- diff --git a/kernel/stackleak.c b/kernel/stackleak.c index ce161a8e8d97..135866ca8878 100644 --- a/kernel/stackleak.c +++ b/kernel/stackleak.c @@ -48,7 +48,7 @@ int stack_erasing_sysctl(struct ctl_table *table, int write, #define skip_erasing() false #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */ -asmlinkage void notrace stackleak_erase(void) +asmlinkage noinstr void stackleak_erase(void) { /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */ unsigned long kstack_ptr = current->lowest_stack; @@ -102,7 +102,6 @@ asmlinkage void notrace stackleak_erase(void) /* Reset the 'lowest_stack' value for the next syscall */ current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64; } -NOKPROBE_SYMBOL(stackleak_erase); void __used __no_caller_saved_registers notrace stackleak_track_stack(void) { diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c index e9db7dcb3e5f..07688a1c686b 100644 --- a/scripts/gcc-plugins/stackleak_plugin.c +++ b/scripts/gcc-plugins/stackleak_plugin.c @@ -446,6 +446,8 @@ static bool stackleak_gate(void) return false; if (!strncmp(TREE_STRING_POINTER(section), ".meminit.text", 13)) return false; + if (!strncmp(TREE_STRING_POINTER(section), ".noinstr.text", 13)) + return false; } return track_frame_size >= 0;
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