Re: [PATCH] init: use KERNEL_DS when trying to start init process

[Mathias Krause]

On Wed, Jun 8, 2011 at 12:47 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Jun 07, 2011 at 07:00:24PM -0700, Linus Torvalds wrote:
>
>> That said, that commit (it's commit ID 4095b99c09e3d in tglx's tree)
>> predates the "real" BK history too: it's part of the (limited) 2.4.x
>> history that was imported from the release patches into BK at the
>> beginning of the use of BK. So at that point we didn't do indivual
>> commits, it's just the import of the v2.4.3.7 -> v2.4.3.8 patch.
>>
>> But yeah, it's old and crufty. And I agree that usually the correct
>> fix is to remove the set_fs() calls entirely.
>
> I think these days its job is done by start_thread(), which is where we
> switch to USER_DS; it's called by ->load_binary() when it decides it's past
> the point of no return.  However, it would be a good idea to verify that
> all architectures do it there properly and we are not exposing a hole by
> removal of this set_fs()...

I've checked all implementations of start_thread() and found some candidates:

SPARC, TILE and Xtensa don't call set_fs(USER_DS), albeit have
different definitions for USER_DS and KERNEL_DS. So those might need
fixing. I'm not familiar with those architectures, so someone else has
to answer this.

Score does not call set_fs(USER_DS) either but that's no problem
because USER_DS has the same value as KERNEL_DS on this architecture.

All others call set_fs(USER_DS) as almost the very first instruction
in start_thread(), or, as for MIPS, do it by setting addr_limit
directly.

Generally, I think, we should get Acks for the questionable arch
maintainers before commiting the patch that removes the call to
set_fs() in search_binary_handler().

I've also checked all binary format handlers if they all call
start_thread() and found a few that do not (binfmt_em86, binfmt_misc
and binfmt_script). But those are just interpreter warppers, i.e. call
search_binary_handler() in the end so should be safe.

Mathias
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