RE: Writing to a const pointer: is this supposed to happen?

Kars Mulder
2020-07-03T13:23:44Z

> There ought to be one that returns a pointer to the first character
> that isn't converted - but I'm no expert on the full range of these
> functions.

I've searched for a function that parses an int from a string and
stores a pointer to the end; I can find some function simple_strtoul
that matches this criterion, but it's documented as

"This function has caveats. Please use kstrtoul instead."

... and kstrtoul does not store a pointer to the end. The documentation
of kstrtoul describes simple_strtoul as obsolete as well. Also, there's
no simple_strtou16 function.

It seems that the standard C function strtoul has the behaviour you
describe as well, but this function is not defined in the kernel except
for certain specific architectures.

> The problem with strdup() is you get the extra (unlikely) failure path.
> 128 bytes of stack won't be a problem if the function is (essentially)
> a leaf.
> Deep stack use is actually likely to be in the bowels of printf())
> inside an obscure error path.

The function already makes a call to kcalloc, so the unlikely out-of-
memory error path already exists; a second memory allocation just
makes it slightly less unlikely. The two new out-of-memory conditions
do happen at different points of the function though, making them
have different side effects. I could fix this by moving my code.

As for this function being a leaf: keep in mind that this function has
the potential of calling printk in an obscure error condition (the user-
provided parameter being longer that 128 characters); quirks_param_set
calls param_set_copystring, which on its turn calls pr_err, which is a
macro for printk.

Meanwhile, here's a patch for copying the parameter to the stack:

---
drivers/usb/core/quirks.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/core/quirks.c b/drivers/usb/core/quirks.c
index e0b77674869c..86b1a6739b4e 100644
--- a/drivers/usb/core/quirks.c
+++ b/drivers/usb/core/quirks.c
@@ -12,6 +12,8 @@
#include <linux/usb/hcd.h>
#include "usb.h"

+#define QUIRKS_PARAM_SIZE 128
+
struct quirk_entry {
u16 vid;
u16 pid;
@@ -23,19 +25,21 @@ static DEFINE_MUTEX(quirk_mutex);
static struct quirk_entry *quirk_list;
static unsigned int quirk_count;

-static char quirks_param[128];
+static char quirks_param[QUIRKS_PARAM_SIZE];

-static int quirks_param_set(const char *val, const struct kernel_param *kp)
+static int quirks_param_set(const char *value, const struct kernel_param *kp)
{
+ char val[QUIRKS_PARAM_SIZE];
char *p, *field;
u16 vid, pid;
u32 flags;
size_t i;
int err;

- err = param_set_copystring(val, kp);
+ err = param_set_copystring(value, kp);
if (err)
return err;
+ strscpy(val, value, sizeof(val));

mutex_lock(&quirk_mutex);

--
2.27.0