Re: [9fans] pngread: alloc chunk's length

["adr via 9fans" <9fans@9fans.net>]

On Mon, 12 Jul 2021, ori@eigenstate.org wrote:
> Why not make getchunk allocate? Somethign like:
> 
> ---
> //.git/fs/object/e8259861da3a55c03491904e4d11c5c15b7577c5/tree/sys/src/cmd/jpg/readpng.c
> +++ sys/src/cmd/jpg/readpng.c
> @@ -94,7 +94,7 @@
> }
>
> static int
> -getchunk(Biobuf *b, char *type, uchar *d, int m)
> +getchunk(Biobuf *b, char *type, uchar **d)
> {
>        uchar buf[8];
>        ulong crc = 0, crc2;
> @@ -103,11 +103,10 @@
>        if(Bread(b, buf, 8) != 8)
>                return -1;
>        n = get4(buf);
> +       *d = pngmalloc(n, 0);
>        memmove(type, buf+4, 4);
>        type[4] = 0;
> -       if(n > m)
> -               sysfatal("getchunk needed %d, had %d", n, m);
> -       nr = Bread(b, d, n);
> +       nr = Bread(b, *d, n);
>        if(nr != n)
>                sysfatal("getchunk read %d, expected %d", nr, n);
>        crc = blockcrc(crctab, crc, type, 4);
> @@ -131,7 +130,7 @@
>        Again:
>                z->p = z->buf;
>                z->e = z->p;
> -               n = getchunk(z->io, type, z->p, IDATSIZE);
> +               n = getchunk(z->io, type, &z->p);
>                if(n < 0 || strcmp(type, "IEND") == 0)
>                        return -1;
>                z->e = z->p + n;
>
>

Hi Ori,

I thought that it would be a good idea to have a separate function
to just get the size without squeezing the fs through all the chunk,
it could be useful in the future. getchunk() could call it, but I
didn't see the point. I imagine that this is not a complete patch
but a suggestion, but anyway remember that you don't need z->buf
any more, and don't forget to free z->p for the next iteration.

By the way after revisiting readpng.c I noticed that I checked for
a negative value of the IHDR chunk length, better do like in the
original code:

[...]
        n = chunklen(b);
        if(n < 13)
                sysfatal("missing IHDR chunk");
[...]

Regards,
adr.

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