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SubjectRe: [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.

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