On Wed, Mar 13, 2024 at 05:45:10AM -0500, G. Branden Robinson wrote: > Hi Alex,
Hi Branden,
> At 2024-03-13T11:21:55+0100, Alejandro Colomar wrote:
> > And use it where the same logic was being open-coded.
> >
> > While at it, fix the logic, which was incorrect in the open-coded call
> > sites, since for an input of 1, it produced 3, but the first prime is 2.
>
> Are you sure? That sounds like the bug I already fixed (but have not
> yet pushed).
I didn't write an exploit, as that would be a little bit more work
(including learning what is indxbib at all).
But by reading the code, I'd say yes.
$ git show master:src/utils/indxbib/indxbib.cpp \
| grepc main \
| grep -C5 is_prime;
int requested_hash_table_size;
check_integer_arg('h', optarg, 1, &requested_hash_table_size);
hash_table_size = requested_hash_table_size;
if ((hash_table_size > 2) && (hash_table_size % 2) == 0)
hash_table_size++;
while (!is_prime(hash_table_size))
hash_table_size += 2;
if (hash_table_size != requested_hash_table_size)
warning("requested hash table size %1 is not prime: using %2"
" instead", optarg, hash_table_size);
}
Let's imagine 'requested_hash_table_size' is 1. check_integer_arg()
should let it pass. Then it's not >2, so we skip the ++. And then it's
not a prime, so we +=2, and then have 3.
BTW, that's the reason I was first confused and suggested raising the
minimum to 3. I hadn't really understood what the code was trying to
do, and only knew what it was actually doing (after a quick read of the
code), and since an input of 1 resulted in an output of 3, I thought
raising the bar to 3 should be ok. It seems not. :)
>
> My working copy, complete with the following debugging output, is
> attached.
>
> $ ./build/indxbib -h 1
> ./build/indxbib: fatal error: argument to -h must not be less than 2
> $ ./build/indxbib -h 2
> ./build/indxbib: debug: using hash table size of 2
> ./build/indxbib: fatal error: no files and no -f option
> $ ./build/indxbib -h 3
> ./build/indxbib: debug: using hash table size of 3
> ./build/indxbib: fatal error: no files and no -f option
> $ ./build/indxbib -h 4
> ./build/indxbib: warning: requested hash table size 4 is not prime: using 5
> instead
> ./build/indxbib: debug: using hash table size of 5
> ./build/indxbib: fatal error: no files and no -f option
>
> > Also, since this is a library function, let's behave well for an input
> > of 0, and return also the first prime, 2.
>
> I'm skeptical of adding a library function for this purpose (finding the
> smallest prime number equal to or greater than the input integer) when
> it _has_ no other call sites.
You had is_prime() with the same exact number of call sites: 2.
If you want to misbehave for an input of 0 from a library call, by
adding an assertion, well that's not too bad, since you're the only
user. In fact, is_prime() has such an assertion, even being a library
call. If you want me to add it, just let me know.
> In fact, the only other caller of is_prime() itself is in libbib, where
> it's not operating on a user-supplied parameter (as in a user of the
> running process).
>
> https://git.savannah.gnu.org/cgit/groff.git/tree/src/libs/libbib/index.cpp?h=1.23.0#n603
>
> Wouldn't it be more idiomatic C++ to migrate libbib to an STL container?
*Mis*quoting someone (was it Torvalds who wrote the Linux coding style?
I guess he was):
First off, I'd suggest printing out a copy of the _C++_
standard, and NOT read it. Burn it, it's a great symbolic
gesture.
I don't like idiomatic C++. In fact, the only thing I like from C++ is
that I can write idiomatic C in it (or mostly; some things don't
compile, like using VLA notation in function parameters that are
pointers).
> (Not that I'm saying you should do that.)
Nice. :)
> Regards,
> Branden
Have a lovely day!
Alex
--
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.
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