Re: [PATCH] 2.5 fast poll on ppc64

Mikael Pettersson
2002-12-26T12:48:54Z

Anton Blanchard writes:
> I was unable to boot 2.5-BK on ppc64 and narrowed it down to the fast
> poll patch. I found:
>
> offsetof(struct poll_list, entries) == 12 but
> sizeof(struct poll_list) == 16
>
> This means pp+1 did not match up with pp->entries. Im not sure what the
> alignment requirements are for a zero length struct (ie is this a
> compiler bug) but the following patch fixes the problem and also changes
> ->len to a long to ensure 8 byte alignment of ->entries on 64bit archs.
>
> Anton
>
> ===== fs/select.c 1.15 vs edited =====
> --- 1.15/fs/select.c Sat Dec 21 20:42:41 2002
> +++ edited/fs/select.c Thu Dec 26 17:31:16 2002
> @@ -362,7 +362,7 @@
>
> struct poll_list {
> struct poll_list *next;
> - int len;
> + long len;
> struct pollfd entries[0];
> };

To me (I'm a compiler writer) it looks like your compiler did NOT
mess up. Assuming struct pollfd has 32-bit alignment, the compiler
is doing the right thing by starting entries[] at offset 12.
The 16-byte size for struct poll_list is because the 'next' field
has 64-bit alignment, which forces the compiler to pad struct
poll_lists's size to a multiple of 8 bytes, i.e. 16 bytes in this
case. So the compiler is not broken.

>
> @@ -471,7 +471,7 @@
> walk->next = pp;
>
> walk = pp;
> - if (copy_from_user(pp+1, ufds + nfds-i,
> + if (copy_from_user(pp->entries, ufds + nfds-i,
> sizeof(struct pollfd)*pp->len)) {

But the old code which assumed pp+1 == pp->entries is so horribly
broken I can't find words for it. s/pp+1/pp->entries/ is the correct fix.

/Mikael