On Wed, 19 Dec 2007 01:14:33 +0100
Eric Dumazet <[email protected]> wrote:
> groups_sort() can be quite long if user loads a large gid table.
>
> This is because GROUP_AT(group_info, some_integer) uses an integer divide.
> So having to do XXX thousand divides during one syscall can lead to very high
> latencies. (NGROUPS_MAX=65536)
>
> In the past (25 Mar 2006), an analog problem was found in groups_search()
> (commit d74beb9f33a5f16d2965f11b275e401f225c949d ) and at that time I changed
> some variables to unsigned int.
>
> I believe that a more generic fix is to make sure NGROUPS_PER_BLOCK is unsigned.
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ac3d496..725a491 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -775,7 +775,7 @@ static inline int above_background_load(void)
>
> struct io_context; /* See blkdev.h */
> #define NGROUPS_SMALL 32
> -#define NGROUPS_PER_BLOCK ((int)(PAGE_SIZE / sizeof(gid_t)))
That was a dopey thing to do. Both PAGE_SIZE and sizeof() are unsigned and
we went and cast it to a signed thing, even though it is a nonsense to ever
consider a negative value of this.
> +#define NGROUPS_PER_BLOCK ((unsigned int)(PAGE_SIZE / sizeof(gid_t)))
> struct group_info {
> int ngroups;
> atomic_t usage;
> diff --git a/kernel/sys.c b/kernel/sys.c
> index d1fe71e..091e58f 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1148,7 +1148,7 @@ static int groups_to_user(gid_t __user *grouplist,
> int count = group_info->ngroups;
>
> for (i = 0; i < group_info->nblocks; i++) {
> - int cp_count = min(NGROUPS_PER_BLOCK, count);
> + int cp_count = min_t(int, NGROUPS_PER_BLOCK, count);
> int off = i * NGROUPS_PER_BLOCK;
> int len = cp_count * sizeof(*grouplist);
>
> @@ -1168,7 +1168,7 @@ static int groups_from_user(struct group_info *group_info,
> int count = group_info->ngroups;
>
> for (i = 0; i < group_info->nblocks; i++) {
> - int cp_count = min(NGROUPS_PER_BLOCK, count);
> + int cp_count = min_t(int, NGROUPS_PER_BLOCK, count);
> int off = i * NGROUPS_PER_BLOCK;
> int len = cp_count * sizeof(*grouplist);
>
We now have a jumble of signed and unsigned, stuck together with min_t
sticky tape.
Please, take a closer look in there and see if there's anything which
*needs* to be signed: is there anything which can actually, sensibly have a
negative quantity? I bet there isn't, and I bet the code would be cleaner,
faster and more obviously correct if it was converted to unsigned throughout.
I blame C. Negative quantities are rare, and C's default of treating
scalars as signed was a mistake. Oh well.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
