linux-kernel - Re: [RFC v2] binfmt_misc: pass binfmt

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Date:   Wed, 15 Jan 2020 13:55:27 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Laurent Vivier <laurent@...ier.eu>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC v2] binfmt_misc: pass binfmt_misc flags to the interpreter

On Wed, Jan 15, 2020 at 01:19:16PM +0100, Laurent Vivier wrote:
> Le 07/01/2020 à 15:50, Laurent Vivier a écrit :
> > Hi,
> > 
> > this change is simple, easy to read and understand but it is really
> > needed by user space application interpreter to know the status of the
> > system configuration.
> > 
> > Could we have a comment saying if there is a problem or if it is good to
> > be merged?
> 
> Anyone?

	FWIW, one thing that looks worrying here is that these bits become
userland ABI after this patch - specific values passed in that thing
can't be changed.  And no a single mention of that in fs/binfmt_misc.c,
leaving a nasty trap.  As far as one can tell, their values are fair game
for reordering, etc. - not even visible outside of fs/binfmt_misc.c;
purely internal constants.  And the effect of such modifications after
your patch will not be "everything breaks, patch gets caught by somebody's
tests" - it will be a quiet breakage for some users.

> >>  #define MISC_FMT_OPEN_BINARY (1 << 30)
> >>  #define MISC_FMT_CREDENTIALS (1 << 29)
> >>  #define MISC_FMT_OPEN_FILE (1 << 28)
> >> +#define MISC_FMT_FLAGS_MASK (MISC_FMT_PRESERVE_ARGV0 | MISC_FMT_OPEN_BINARY | \
> >> +			     MISC_FMT_CREDENTIALS | MISC_FMT_OPEN_FILE)

IOW, you are making those parts of userland ABI cast in stone forever.
Whether this bit assignment does make sense or not, such things really
should not be hidden.