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Re: [PATCH] Fix p{readv,writev}{64} consolidation implementation

On 15/06/2016 15:21, Chris Metcalf wrote:
> On 6/15/2016 9:45 AM, Adhemerval Zanella wrote:
>> On 15/06/2016 02:37, Mike Frysinger wrote:
>>> On 14 Jun 2016 18:54, Adhemerval Zanella wrote:
>>>> This patch fixes the p{readv,writev}{64} consolidation implementation
>>>> from commits 4e77815 and af5fdf5.  Different from pread/pwrite
>>>> implementation, preadv/pwritev implementation does not require
>>>> __ALIGNMENT_ARG because kernel syscall prototypes define
>>>> the high and low part of the off_t, if it is the case, directly
>>>> (different from pread/pwrite where the architecture ABI for passing
>>>> 64-bit values must be in consideration for passsing the arguments).
>>> i had looked at that specifically but thought it ok because the old code
>>> was using the alignment arg.  was the old code broken too ?
>>> this is what the preadv code looked like:
>>> -ssize_t
>>> -__libc_preadv (int fd, const struct iovec *vector, int count, off_t offset)
>>> -{
>>> -  assert (sizeof (offset) == 4);
>>> -  return SYSCALL_CANCEL (preadv, fd,
>>> -                         vector, count, __ALIGNMENT_ARG
>>> -                         __LONG_LONG_PAIR (offset >> 31, offset));
>>> -}
>>> although i guess this isn't too surprising as this code was in the
>>> generic sysdeps dir which currently doesn't have as many users as
>>> we wish it did :).
>> I though it too, but before the consolidation patch only nios2 and tile 32-bits
>> used the generic preadv.c implementation.  And only tile 32-bits defines
>> __ASSUME_ALIGNED_REGISTER_PAIRS (so __ALIGNMENT_ARG will pad the argument with 0).
>> However since the syscall is defined as in linux source:
>> fs/read_write.c:
>>   991 SYSCALL_DEFINE5(preadv, unsigned long, fd, const struct iovec __user *, vec,
>>   992                 unsigned long, vlen, unsigned long, pos_l, unsigned long, pos_h)
>>   993 {
>>   994         loff_t pos = pos_from_hilo(pos_h, pos_l);
>>   995
>>   996         return do_preadv(fd, vec, vlen, pos, 0);
>>   997 }
> Actually, the relevant code at fs/read_write.c:1162 is:
> COMPAT_SYSCALL_DEFINE5(preadv, compat_ulong_t, fd,
>         const struct compat_iovec __user *,vec,
>         compat_ulong_t, vlen, u32, pos_low, u32, pos_high)
> {
>     loff_t pos = ((loff_t)pos_high << 32) | pos_low;
>     return do_compat_preadv64(fd, vec, vlen, pos, 0);
> }
> but it amounts to the same thing as far the arguments are concerned.

This is for 64-bits kernel that provides the 32-bit compat syscalls.
For 32-bits kernels only it the code I referenced above.

>> The idea is not really to align the argument to zero pass, but rather to split
>> the possible 64-bits argument in high and low as required (as the default
>> implementation was doing [2]). On tile, it is working because the preadv.c
>> offset is 32-bits and thus the high word is indeed zero, but it is passing
>> one superfluous argument.
> No, what happens is that instead of passing r3=pos_low=lo, r4=pos_high=0, we
> are passing r3=pos_low=0, r4=pos_high=lo, r5=0.  This means that we get a crazy
> high offset and either fail or (for a sufficiently large file) get the wrong data.
> Filed as bug 20261.

I was referring to *old* behaviour (pre-consolidation) implementation 
(the sysdeps/unix/sysv/linux/generic/wordsize-32/preadv.c), which did:

  return SYSCALL_CANCEL (preadv, fd,
                         vector, count, __ALIGNMENT_ARG
                         __LONG_LONG_PAIR (offset >> 31, offset));

It has 2 issue:

 1. It passed one superfluous argument to preadv. On tilepro build
    I noted that is using internal_syscall6, which means it is passing
    { fd, vector, cound, 0, offset, offset>>31 }.  It is not wrong,
    since for this code off_t will be the old 32-bit value, but the
    semantic is wrong.

 2. __LONG_LONG_PAIR is not correct for big-endian.

Now for BZ#20261 I do not think it applicable since this is a fix for
consolidation done in development phase, it does not appear in any
released version.

Now, your analysis is correct for *current* code and it is contains the
__LONG_LONG_PAIR issue due the SYSCALL_LL usage.  I will change it
and post a second version.

> It appears to be true for both preadv and pwritev, though I only tested preadv.
>> Also my understanding is this generic implementation
>> does work correctly in every architecture because __LONG_LONG_PAIR relies
>> on endianess.
> In fact that's another bug; __LONG_LONG_PAIR is intended only to
> be used if you are simulating passing a 64-bit value in 32-bit registers,
> where the ABI naturally splits the 64 bits into a high and low part
> according to endianness.
> In this example we are calling into the kernel where it expects a pos_low
> and a pos_high in a particular order.  On a big-endian machine, the
> __LONG_LONG_PAIR will put the high part first and the kernel will
> see the wrong offset.

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