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SubjectRe: [PATCH] HDLC patch for 2.5.5 (0/3)
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Francois Romieu <romieu@cogenit.fr> writes:

> I agree there's a way for an application to cause binary incompatibility if
> it does:
>
> struct userspace_foo {
> struct if_settings frob;
> int nitz;
> } bar;
>
> If size of struct if_settings changes (increases OR decreases), access to
> bar.nitz doesn't work as expected.

I assumed it's union and not a struct, you're right.

> But:
> in hdlc_xxx_ioctl, only knowledge of the protocol-related member of the
> union
> hdlcs_hdlcu is required. Nowhere does the code depend on size of if_settings.

I see now, t seems I haven't read the patches carefully enough.

Now... You just want to introduce an artificial struct which contains
only the union... Why? We could use just the union instead (?).

struct hdlc_settings {
union {
/* sync_serial_settings removed */
raw_hdlc_proto raw_hdlc;
cisco_proto cisco;
fr_proto fr;
fr_proto_pvc fr_pvc;
te1_settings te1;
} hdlcs_hdlcu;
};

Still, te1_settings are interface-related :-) Ok, I assume it goes
to the following:

> include/linux/whatever/ioctl.h:
> [...]
> struct whatever_settings {
> union {
> /* sync_serial_settings is back */
> sync_serial_settings sync;
> fancy_settings fancy;
> }
> };
>
> include/linux/if.h:
> [...]
> struct if_settings
> {
> unsigned int type; /* Type of physical device or protocol */
> union {
> struct hdlc_settings ifsu_hdlc;
> struct whatever_settings ifsu_whatever;
> } ifs_ifsu;
> };
>
> As long as the application only accesses its data and doesn't try to embed
> the variable sized kernel structure into its own, it won't break here either.

Yes, the compiler would compile that. Anyway, don't you think it's
a little messy? Void * pointers are IMHO not that evil.

Not sure about that, I have to think on it...
--
Krzysztof Halasa
Network Administrator
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