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SubjectRe: [PATCH] iov_iter: separate direction from flavour
Hi,

On Thu, Apr 22, 2021 at 02:50:39PM -0400, Al Viro wrote:
> Instead of having them mixed in iter->type, use separate ->iter_type
> and ->data_source (u8 and bool resp.) And don't bother with (pseudo-)
> bitmap for the former - microoptimizations from being able to check
> if the flavour is one of two values are not worth the confusion for
> optimizer. It can't prove that we never get e.g. ITER_IOVEC | ITER_PIPE,
> so we end up with extra headache.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

This patch results in the following runtime warning on nommu systems.

[ 8.567154] Run /init as init process
[ 8.572112] ------------[ cut here ]------------
[ 8.572248] WARNING: CPU: 0 PID: 1 at lib/iov_iter.c:468 iov_iter_init+0x35/0x58
[ 8.572484] CPU: 0 PID: 1 Comm: init Not tainted 5.13.0-09606-g303392fd5c16 #1
[ 8.572695] Hardware name: MPS2 (Device Tree Support)
[ 8.573278] [<2100ae75>] (unwind_backtrace) from [<2100a2bb>] (show_stack+0xb/0xc)
[ 8.573594] [<2100a2bb>] (show_stack) from [<2100da03>] (__warn+0x5f/0x80)
[ 8.573738] [<2100da03>] (__warn) from [<2100da55>] (warn_slowpath_fmt+0x31/0x60)
[ 8.573886] [<2100da55>] (warn_slowpath_fmt) from [<210d8e1d>] (iov_iter_init+0x35/0x58)
[ 8.574044] [<210d8e1d>] (iov_iter_init) from [<21059cab>] (vfs_read+0x89/0xc6)
[ 8.574191] [<21059cab>] (vfs_read) from [<2105d92b>] (read_code+0x15/0x2e)
[ 8.574329] [<2105d92b>] (read_code) from [<21085a8d>] (load_flat_file+0x341/0x4f0)
[ 8.574481] [<21085a8d>] (load_flat_file) from [<21085e03>] (load_flat_binary+0x47/0x2dc)
[ 8.574639] [<21085e03>] (load_flat_binary) from [<2105d581>] (bprm_execve+0x1fd/0x32c)
[ 8.574797] [<2105d581>] (bprm_execve) from [<2105dbb3>] (kernel_execve+0xa3/0xac)
[ 8.574947] [<2105dbb3>] (kernel_execve) from [<211e7095>] (kernel_init+0x31/0xb0)
[ 8.575099] [<211e7095>] (kernel_init) from [<2100814d>] (ret_from_fork+0x11/0x24)
[ 8.575287] Exception stack(0x21429fb0 to 0x21429ff8)
[ 8.575433] 9fa0: 00000000 00000000 00000000 00000000
[ 8.575593] 9fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[ 8.575743] 9fe0: 00000000 00000000 00000000 00000000 00000000 00000000
[ 8.575933] ---[ end trace ba15568c05035a77 ]---

This is with qemu's mps2-an385 emulation and and mps2_defconfig.
The same warning is also observed with m68k and mcf5208evb,
though the traceback isn't as nice.

WARNING: CPU: 0 PID: 1 at lib/iov_iter.c:468 0x40135e4e
...
Call Trace:
[<402b0f42>] 0x402b0f42
[<402b0fea>] 0x402b0fea
[<40135e4e>] 0x40135e4e
[<40135e4e>] 0x40135e4e
[<4009c610>] 0x4009c610
...

Reverting this patch fixes the problem for both mps2-an385 and mcf5208evb.

Guenter

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