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    SubjectRe: [PATCH V11 4/5] vsprintf: add printk specifier %px
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    On Thu, 2017-11-30 at 10:26 +1100, Tobin C. Harding wrote:
    > On Wed, Nov 29, 2017 at 03:20:58PM -0800, Andrew Morton wrote:
    > > On Wed, 29 Nov 2017 13:05:04 +1100 "Tobin C. Harding" <me@tobin.cc> wrote:
    > >
    > > > printk specifier %p now hashes all addresses before printing. Sometimes
    > > > we need to see the actual unmodified address. This can be achieved using
    > > > %lx but then we face the risk that if in future we want to change the
    > > > way the Kernel handles printing of pointers we will have to grep through
    > > > the already existent 50 000 %lx call sites. Let's add specifier %px as a
    > > > clear, opt-in, way to print a pointer and maintain some level of
    > > > isolation from all the other hex integer output within the Kernel.
    > > >
    > > > Add printk specifier %px to print the actual unmodified address.
    > > >
    > > > ...
    > > >
    > > > +Unmodified Addresses
    > > > +====================
    > > > +
    > > > +::
    > > > +
    > > > + %px 01234567 or 0123456789abcdef
    > > > +
    > > > +For printing pointers when you _really_ want to print the address. Please
    > > > +consider whether or not you are leaking sensitive information about the
    > > > +Kernel layout in memory before printing pointers with %px. %px is
    > > > +functionally equivalent to %lx. %px is preferred to %lx because it is more
    > > > +uniquely grep'able. If, in the future, we need to modify the way the Kernel
    > > > +handles printing pointers it will be nice to be able to find the call
    > > > +sites.
    > > > +
    > >
    > > You might want to add a checkpatch rule which emits a stern
    > > do-you-really-want-to-do-this warning when someone uses %px.
    > >
    >
    > Oh, nice idea. It has to be a CHECK but right?

    No, it has to be something that's not --strict
    so a WARN would probably be best.

    > By stern, you mean use stern language?

    I hope he doesn't mean tweet.

    Something like:
    ---
    scripts/checkpatch.pl | 31 +++++++++++++++++++++++++------
    1 file changed, 25 insertions(+), 6 deletions(-)

    diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
    index 0ce249f157a1..9d789cbe7df5 100755
    --- a/scripts/checkpatch.pl
    +++ b/scripts/checkpatch.pl
    @@ -5758,21 +5758,40 @@ sub process {
    defined $stat &&
    $stat =~ /^\+(?![^\{]*\{\s*).*\b(\w+)\s*\(.*$String\s*,/s &&
    $1 !~ /^_*volatile_*$/) {
    + my $complete_extension = "";
    + my $extension = "";
    my $bad_extension = "";
    my $lc = $stat =~ tr@\n@@;
    $lc = $lc + $linenr;
    + my $stat_real;
    for (my $count = $linenr; $count <= $lc; $count++) {
    my $fmt = get_quoted_string($lines[$count - 1], raw_line($count, 0));
    $fmt =~ s/%%//g;
    - if ($fmt =~ /(\%[\*\d\.]*p(?![\WFfSsBKRraEhMmIiUDdgVCbGNO]).)/) {
    - $bad_extension = $1;
    - last;
    + while ($fmt =~ /(\%[\*\d\.]*p(\w))/g) {
    + $complete_extension = $1;
    + $extension = $2;
    + if ($extension !~ /[FfSsBKRraEhMmIiUDdgVCbGNOx]/) {
    + $bad_extension = $complete_extension;
    + last;
    + }
    + if ($extension eq "x") {
    + if (!defined($stat_real)) {
    + $stat_real = raw_line($linenr, 0);
    + for (my $count = $linenr + 1; $count <= $lc; $count++) {
    + $stat_real = $stat_real . "\n" . raw_line($count, 0);
    + }
    + }
    + WARN("VSPRINTF_POINTER_PX",
    + "Using vsprintf pointer extension '$complete_extension' exposes kernel address for possible hacking\n" . "$here\n$stat_real\n");
    + }
    }
    }
    if ($bad_extension ne "") {
    - my $stat_real = raw_line($linenr, 0);
    - for (my $count = $linenr + 1; $count <= $lc; $count++) {
    - $stat_real = $stat_real . "\n" . raw_line($count, 0);
    + if (!defined($stat_real)) {
    + $stat_real = raw_line($linenr, 0);
    + for (my $count = $linenr + 1; $count <= $lc; $count++) {
    + $stat_real = $stat_real . "\n" . raw_line($count, 0);
    + }
    }
    WARN("VSPRINTF_POINTER_EXTENSION",
    "Invalid vsprintf pointer extension '$bad_extension'\n" . "$here\n$stat_real\n");
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