linux-kernel - Re: [PATCH v2] soc: qcom: cmd-db: replace deprecated strncpy with strtomem

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Message-ID: <CAFhGd8rdd=qmAmax27gvExZ5sweC=porkS6dW3RJh7NV1ydjSw@mail.gmail.com>
Date: Tue, 19 Mar 2024 14:10:23 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Bjorn Andersson <andersson@...nel.org>
Cc: Konrad Dybcio <konrad.dybcio@...aro.org>, linux-arm-msm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] soc: qcom: cmd-db: replace deprecated strncpy with strtomem

Hi,

On Mon, Mar 18, 2024 at 8:37 PM Bjorn Andersson <andersson@...nel.org> wrote:
>
> On Mon, Mar 18, 2024 at 10:49:23PM +0000, Justin Stitt wrote:
> > strncpy() is deprecated for use on NUL-terminated destination strings
> > [1] and as such we should prefer more robust and less ambiguous string
> > interfaces.
> >
>
> I don't mind changing the strncpy() in this function, but I don't think
> this problem description adequately describes the problem you're
> solving.
>
> If the motivation is that we want 0 users of strncpy() in the kernel,
> then say so.

Fair. You caught me in a bad case of "copy pasting this blurb into all
my patches". You are right though, the true motivation here is to rid
the kernel of strncpy.

>
> > @query is already marked as __nonstring and doesn't need to be
> > NUL-terminated.
>
> You're not wrong, but in the event that strlen(id) < sizeof(ent->id) the
> destination should be NUL-padded - exactly one of the well known,
> normally unwanted, effects of strncpy(). strtomem() does explicitly not
> do this.
>
> > Since @id is a string, we can use the self-describing
> > string API strtomem().
>
> "self-describing"?
>

In the sense that its name matches its functionality:

strncpy    === string to string copy, bounded by n

strtomem === string to memory buffer

strncpy technically does the latter functionality as well but it may
not be obvious in all cases that the destination buffer is not a
string. Granted, in this case, it is extremely obvious what the
behavior is because query is marked nonstring.

> >
> > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.enhtml [2]
> > Link: https://github.com/KSPP/linux/issues/90
> > Cc: linux-hardening@...r.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > Changes in v2:
> > - use strtomem instead of memcpy (thanks Kees)
> > - Link to v1: https://lore.kernel.org/r/20240314-strncpy-drivers-soc-qcom-cmd-db-c-v1-1-70f5d5e70732@google.com
> > ---
> > Note: build-tested only.
> >
> > Found with: $ rg "strncpy\("
> > ---
> >  drivers/soc/qcom/cmd-db.c | 9 ++-------
> >  1 file changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> > index a5fd68411bed..d05f35d175bd 100644
> > --- a/drivers/soc/qcom/cmd-db.c
> > +++ b/drivers/soc/qcom/cmd-db.c
> > @@ -141,18 +141,13 @@ static int cmd_db_get_header(const char *id, const struct entry_header **eh,
> >       const struct rsc_hdr *rsc_hdr;
> >       const struct entry_header *ent;
> >       int ret, i, j;
> > -     u8 query[sizeof(ent->id)] __nonstring;
> > +     u8 query[sizeof(ent->id)] __nonstring = { 0 };
> >
> >       ret = cmd_db_ready();
> >       if (ret)
> >               return ret;
> >
> > -     /*
> > -      * Pad out query string to same length as in DB. NOTE: the output
> > -      * query string is not necessarily '\0' terminated if it bumps up
> > -      * against the max size. That's OK and expected.
> > -      */
> > -     strncpy(query, id, sizeof(query));
> > +     strtomem(query, id);
>
> query needs to be NUL-padded to sizeof(ent->id) bytes (like strncpy
> does), something you recognized by adding the zero-initialization above.
> But why split this requirement across two non-adjacent lines? Isn't this
> what strtomem_pad() is supposed to do?

Yes, strtomem_pad() will accomplish this task. I'll send a v3 fixing
up the commit log and use the pad version.

>
> Regards,
> Bjorn
>
> >
> >       for (i = 0; i < MAX_SLV_ID; i++) {
> >               rsc_hdr = &cmd_db_header->header[i];
> >
> > ---
> > base-commit: fe46a7dd189e25604716c03576d05ac8a5209743
> > change-id: 20240314-strncpy-drivers-soc-qcom-cmd-db-c-284f3abaabb8
> >
> > Best regards,
> > --
> > Justin Stitt <justinstitt@...gle.com>
> >

Thanks
Justin