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SubjectRe: [PATCH] Reduce stack usage in module.c
On Tue, 29 Mar 2005 09:43:12 -0800, Randy.Dunlap <rddunlap@osdl.org> wrote:
> Yum Rayan wrote:
> > Attempt to reduce stack usage in module.c (linux-2.6.12-rc1-mm3).
> > Specifically from checkstack.pl
> >
> > Before patch
> > ------------------
> > who_is_doing_it: 512
> > obsolete_params: 160
> >
> > After patch
> > ----------------
> > who_is_doing_it: none
> So all function local variables are in registers?

Yes, all function local variables of the patched who_is_doing_it(...)
are in registers.

> > Also while at it, fix following in who_is_doing_it(...)
> > - use only as much memory is needed
> > - do not write past array index for the boundary case
>
> I don't see a boundary case problem with the current code,
> hence I don't see why the kmalloc(len + 1, GFP_KERNEL) is
> needed...

Let's consider the original code and len = 513

1399 static void who_is_doing_it(void)
1400 {
1401 /* Print out all the args. */
1402 char args[512];
1403 unsigned long i, len = current->mm->arg_end -
current->mm->arg_start;
1404
1405 if (len > 512)
1406 len = 512;
1407
1408 len -= copy_from_user(args, (void
*)current->mm->arg_start, len);
1409
1410 for (i = 0; i < len; i++) {
1411 if (args[i] == '\0')
1412 args[i] = ' ';
1413 }
1414 args[i] = 0;
1415 printk("ARGS: %s\n", args);
1416 }

After lines 1410 thru 1413, "i" wil be 512. So line 1414 will be
"args[512] = 0". But args is 512 byte array with last legally
accessible element at 511?

> File names start one level deeper than wanted. They should begin
> with linux/ or a/ or ./ e.g.
> There are plenty of docs on this, please let me know if you need
> references to them.

Point noted. Will post patch to linux/Documentation/SubmittingPatches,
hopefully making it more clear. Reworked patch at end of email.

>
> > @@ -769,15 +769,25 @@
> > struct kernel_param *kp;
> > unsigned int i;
> > int ret;
> > + char *sym_name = NULL;
> > + unsigned int sym_name_len = 0;
> >
> > kp = kmalloc(sizeof(kp[0]) * num, GFP_KERNEL);
> > if (!kp)
> > return -ENOMEM;
>
> Style thing, I guess, but since the case of num == 0 doesn't do
> anything here, I would just begin the function with:
>
> if (!num)
> return;
> or goto out;
> to maintain one return point.
>
> and then eliminate the kmalloc()s, if (num), kfree()s, and
> parse_args().

Was attempting to preserve the call flow of the previous author. But
yes, this makes more sense. I changed code to return "0" for !num
case.

Thanks,
Rayan

Summary: Reduce stack usage in obsolete_params() and who_is_doing_it()
Target: linux-2.6.12-rc1-mm3
Signed-off-by: Yum Rayan <yum.rayan@gmail.com>

--- a/kernel/module.c 2005-03-25 22:11:06.000000000 -0800
+++ b/kernel/module.c 2005-03-29 22:16:09.000000000 -0800
@@ -767,17 +767,27 @@
const char *strtab)
{
struct kernel_param *kp;
- unsigned int i;
+ char *sym_name;
+ unsigned int sym_name_len, i;
int ret;

+ if (!num)
+ return 0;
+
kp = kmalloc(sizeof(kp[0]) * num, GFP_KERNEL);
if (!kp)
return -ENOMEM;

- for (i = 0; i < num; i++) {
- char sym_name[128 + sizeof(MODULE_SYMBOL_PREFIX)];
+ sym_name_len = 128 + sizeof (MODULE_SYMBOL_PREFIX);
+ sym_name = kmalloc(sym_name_len, GFP_KERNEL);
+ if (!sym_name) {
+ ret = -ENOMEM;
+ goto free_kp;
+ }

- snprintf(sym_name, sizeof(sym_name), "%s%s",
+ for (i = 0; i < num; i++) {
+
+ snprintf(sym_name, sym_name_len, "%s%s",
MODULE_SYMBOL_PREFIX, obsparm[i].name);

kp[i].name = obsparm[i].name;
@@ -791,13 +801,15 @@
printk("%s: falsely claims to have parameter %s\n",
name, obsparm[i].name);
ret = -EINVAL;
- goto out;
+ goto free_sym;
}
kp[i].arg = &obsparm[i];
}

ret = parse_args(name, args, kp, num, NULL);
- out:
+ free_sym:
+ kfree(sym_name);
+ free_kp:
kfree(kp);
return ret;
}
@@ -1399,12 +1411,16 @@
static void who_is_doing_it(void)
{
/* Print out all the args. */
- char args[512];
+ char *args;
unsigned long i, len = current->mm->arg_end - current->mm->arg_start;

if (len > 512)
len = 512;

+ args = kmalloc(len + 1, GFP_KERNEL);
+ if (!args)
+ return;
+
len -= copy_from_user(args, (void *)current->mm->arg_start, len);

for (i = 0; i < len; i++) {
@@ -1413,6 +1429,7 @@
}
args[i] = 0;
printk("ARGS: %s\n", args);
+ kfree(args);
}

/* Allocate and load the module: note that size of section 0 is always
-
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