Re: [PATCH v3 4/6] stdlib: Sync canonicalize with gnulib [BZ #10635] [BZ #26592] [BZ #26341] [BZ #24970]

[Paul Eggert]

On 12/30/20 5:10 AM, Adhemerval Zanella wrote:

> > it is just really
> > a small optimization that adds code complexity on a somewhat convoluted
> > code.

The code is indeed simpler without the NARROW_ADDRESSES optimization, so 
I removed that optimization by installing the attached patch into Gnulib.

> > For ENAMETOOLONG, I think this is the right error code: it enforces
> > that we do not support internal objects longer that PTRDIFF_MAX.

This sounds backwards, as the code returns ENOMEM every other place it 
tries to create an internal object longer than PTRDIFF_MAX - these 
ENOMEM checks are in the malloc calls invoked by scratch_buffer_grow and 
scratch_buffer_grow_preserve. It would be odd for canonicalize_file_name 
to return ENAMETOOLONG for this one particular way of creating a 
too-large object, while at the same time it returns ENOMEM for all the 
other ways.

Besides, ENAMETOOLONG is the POSIX error code for exceeding NAME_MAX or 
PATH_MAX, which is not what is happening here.

In Gnulib and other GNU apps we've long used the tradition that ENOMEM 
means you've run out of memory, regardless of whether it's because your 
heap or your address space is too small. This is a good tradition and 
it'd be good to use it here too.

> > I think it should be a fair assumption to make it on internal code, such
> > as realpath

Yes, staying less than PTRDIFF_MAX is a vital assumption on internal 
objects. I'd go even further and say it's important for user-supplied 
objects, too, as so much code relies on pointer subtraction and we can't 
realistically prohibit that within glibc.

> (this is another reason why I think NARROW_ADDRESSES is not 
> necessary).

Unfortunately, if we merely assume every object has at most PTRDIFF_MAX 
bytes, we still must check for overflow when adding the sizes of two 
objects. The NARROW_ADDRESSES optimization would have let us avoid that 
unnecessary check on 64-bit machines.

> And your fix (from 93e0186d4) does not really solve the issue, since
> now that len is a size_t the overflow check won't catch the potentially
> allocation larger than PTRDIFF_MAX (the realpath will still fail with
> ENOMEM though).

Sure, which means the code is doing the right thing: it's failing with 
ENOMEM because it ran out of memory. There is no need for an extra 
PTRDIFF_MAX check in canonicalize.c if malloc (via scratch_buffer_grow) 
already does the check.
> Wouldn't the below be simpler?
>
>                size_t len = strlen (end);
>                if (len > IDX_MAX || INT_ADD_OVERFLOW ((idx_t) len, n))
>                  {
>                    __set_errno (ENAMETOOLONG);
>                    goto error_nomem;
>                  }

It's not simpler than the attached Gnulib patch, because it contains an 
unnecessary comparison to IDX_MAX and an unnecessary cast to idx_t.

0001-canonicalize-remove-NARROW_ADDRESSES-optimization.patch
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