linux-kernel - Re: [PATCH] [RFC v2] qnx: avoid -Wstringop-overread warning, again

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Date:   Tue, 21 Sep 2021 17:15:33 +0200
From:   Anders Larsen <al@...rsen.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC v2] qnx: avoid -Wstringop-overread warning, again

On Tuesday, 2021-09-21 10:18 Arnd Bergmann wrote:
> On Mon, Sep 20, 2021 at 7:26 PM Linus Torvalds 
<torvalds@...ux-foundation.org> wrote:
> > It sounds like we can avoid the gcc bug if we just always use
> > "de->de_name[]". Then we don't need to depend on magical behavior
> > about one particular gcc version and a strange empty array in front of
> > it.
> >
> > IOW, something like the attached simpler thing that just does that
> > "always use de_name[]" and has a comment about why we don't do the
> > natural thing

well, the code in question actually does not use anything from struct 
qnx4_inode_entry except di_fname and di_status;
they are available at the same offsets in struct qnx4_link_info as well, so 
wouldn't it be even simpler to just always use the fields of the latter 
structure?

Like in the attached patch which replaces b7213ffa0e58?
($me feeling bad for reverting Linus' patch!)

That way, the compiler should never see any access to the (shorter) 
qnx4_inode_entry.di_fname

BTW, in the process I noticed that fs/qnx4/namei.c was missed by 663f4deca76 
back in 2013 and so is still calling strlen() on untrusted data; the second 
part of the patch takes care of that.

> > Also, just what version of gcc is the broken one? You say "gcc-11",
> > but I certainly don't see it with _my_ version of gcc-11, so can we
> > (just for that comment) document more precisely what version you have
> > (or possibly what config you use to trigger it).
> 
> I'm using the gcc-11.1.0 that I uploaded to
> https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/11.1.0/

I don't have that compiler version, so obviously I couldn't test if the patch 
solves the problem.

Cheers
Anders

View attachment "patch.diff" of type "text/x-patch" (4909 bytes)