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Message-ID: <87bmkw5owv.fsf@rasmusvillemoes.dk>
Date: Tue, 24 Oct 2017 21:25:20 +0200
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.com, "Jason A. Donenfeld" <Jason@...c4.com>, Theodore Ts'o <tytso@....edu>, Linus Torvalds <torvalds@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, Paolo Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ker.com>, "Roberts\, William C" <william.c.roberts@...el.com>, Tejun Heo <tj@...nel.org>, Jordan Glover <Golden_Miller83@...tonmail.ch>, Greg KH <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <wilal.deacon@....com>, Steven Rostedt <rostedt@...dmis.org>, Chris Fries <cfries@...gle.com>
Subject: Re: [PATCH v7] printk: hash addresses printed with %p
On Tue, Oct 24 2017, "Tobin C. Harding" <me@...in.cc> wrote:
> +
> +/* Maps a pointer to a 32 bit unique identifier. */
> +static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> +{
> + unsigned int hashval;
> +
> + if (static_branch_unlikely(&no_ptr_secret))
> + return "(pointer value)";
Eh, you probably meant to call
string(buf, end, "(pointer value)", some-appropriate-spec)
otherwise this will either crash very soon (when the following output
wants to overwrite that '(' in .rodata), or at the very least cause a
completely bogus eventual return value (if the "(pointer value)" string
happens to have an address > end, so that we don't actually attempt any
more printing).
Whether the given spec is suitable as some-appropriate-spec or one
should just use a fixed one I don't know.
The rest are just random thoughts/ramblings/questions, feel free to ignore.
Can one do some qemu magic to test the no_ptr_secret code path?
> +
> +#ifdef CONFIG_64BIT
> + hashval = (unsigned int)siphash_1u64((u64)ptr, &ptr_secret);
> +#else
> + hashval = (unsigned int)siphash_1u32((u32)ptr, &ptr_secret);
> +#endif
> +
> + spec.field_width = 2 * sizeof(unsigned int);
> + spec.flags = SMALL;
> + spec.base = 16;
> +
> + return number(buf, end, hashval, spec);
> +}
Maybe include SPECIAL in flags? I know that this is just meant to be
mostly-unique identifier and its not really a number, but it's still
weird to see a string of hex digits not preceded by 0x. Also, maybe use
.precision to get zero-padding instead of spaces?
I haven't followed the discussion too closely, but has it been
considered to exempt NULL from hashing?
Rasmus
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