Adding Linus and some other guys into Cc.
My concern is taking a lock when processing a printf format, see
below for more details.
On Tue 2024-12-17 12:37:09, Rob Herring (Arm) wrote:
> The callers for of_modalias() generally need the module alias as part of
> some larger string. That results in some error prone manipulation of the
> buffer prepend/append the module alias string. In fact,
> of_device_uevent_modalias() has several issues. First, it's off by one
> too few characters in utilization of the full buffer. Second, the error
> paths leave OF_MODALIAS with a truncated value when in the end nothing
> should be added to the buffer. It is also fragile because it needs
> internal details of struct kobj_uevent_env. add_uevent_var() really
> wants to write the env variable and value in one shot which would need
> either a temporary buffer for value or a format specifier.
>
> Fix these issues by adding a new printf format specifier, "%pOFm". With
> the format specifier in place, simplify all the callers of
> of_modalias(). of_modalias() can also be simplified with vsprintf()
> being the only caller as it avoids the error conditions.
>
> Cc: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
> Signed-off-by: Rob Herring (Arm) <robh@xxxxxxxxxx>
> ---
> Documentation/core-api/printk-formats.rst | 1 +
> drivers/of/device.c | 25 ++--------------
> drivers/of/module.c | 35 +++++------------------
> drivers/of/unittest.c | 2 ++
> include/linux/of.h | 8 +++---
> lib/vsprintf.c | 7 +++--
> 6 files changed, 22 insertions(+), 56 deletions(-)
>
> diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
> index ecccc0473da9..d72fe3d8c427 100644
> --- a/Documentation/core-api/printk-formats.rst
> +++ b/Documentation/core-api/printk-formats.rst
> @@ -496,6 +496,7 @@ equivalent to %pOFf.
> - F - device node flags
> - c - major compatible string
> - C - full compatible string
> + - m - module alias string
>
> The separator when using multiple arguments is ':'
>
> diff --git a/drivers/of/device.c b/drivers/of/device.c
> index edf3be197265..ae8c47d5db8e 100644
> --- a/drivers/of/device.c
> +++ b/drivers/of/device.c
> @@ -256,24 +251,10 @@ EXPORT_SYMBOL_GPL(of_device_uevent);
>
> int of_device_uevent_modalias(const struct device *dev, struct kobj_uevent_env *env)
> {
> - int sl;
> -
> if ((!dev) || (!dev->of_node) || dev->of_node_reused)
> return -ENODEV;
>
> - /* Devicetree modalias is tricky, we add it in 2 steps */
> - if (add_uevent_var(env, "MODALIAS="))
> - return -ENOMEM;
> -
> - sl = of_modalias(dev->of_node, &env->buf[env->buflen-1],
> - sizeof(env->buf) - env->buflen);
> - if (sl < 0)
> - return sl;
> - if (sl >= (sizeof(env->buf) - env->buflen))
> - return -ENOMEM;
> - env->buflen += sl;
> -
> - return 0;
> + return add_uevent_var(env, "MODALIAS=%pOFm", dev->of_node);
The proposed %pOFm format takes a lock inside. It calls:
size_t of_modalias(const struct device_node *np, char *str, size_t len)
{
[...]
csize = snprintf(str, len, "of:N%pOFn%c%s", np, 'T',
of_node_get_device_type(np));
[...]
which calls:
+ of_node_get_device_type()
+ of_get_property()
+ of_find_property()
, where
struct property *of_find_property(const struct device_node *np,
const char *name,
int *lenp)
{
[...]
raw_spin_lock_irqsave(&devtree_lock, flags);
pp = __of_find_property(np, name, lenp);
raw_spin_unlock_irqrestore(&devtree_lock, flags);
[...] return pp;
I personally think that taking locks when formatting string is
a way to hell.
In this case, add_uevent_var() is lockless so that it should not
cause any problem. But just imagine that it does:
int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...)
{
some_lock();
va_start(args, format);
len = vsnprintf(&env->buf[env->buflen],
sizeof(env->buf) - env->buflen,
format, args);
va_end(args);
some_unlock();
return 0;
}
Would anyone consider that the vsprintf() here might need to take a lock?
Also, the format might be used in printk(). We put a huge effort into
creating a lockless ringbuffer and safe console locking. I would
really appreciate to avoid any locking in the formatting part.
That said, we already have a precedent. "%pOFf" might take a lock,
for example, via:
+ fwnode_full_name_string()
+ fwnode_handle_put()
+ of_fwnode_put()
+ of_node_put()
+ kobject_put()
+ kref_put()
+ schedule_delayed_work()
+ queue_delayed_work()
+ queue_delayed_work_on()
+ __queue_delayed_work()
+ add_timer_on()
+ add_timer_on()
+ lock_timer_base()
+ raw_spin_lock_irqsave(&base->lock, *flags);
But this would happen only when the last reference is released
when formatting the string which is kind of corner case.
As I said, I think that taking lock in vsprintf() formats is highly
unexpected and thus a way to hell.
What do others think, please?
Best Regards,
Petr
> }
> EXPORT_SYMBOL_GPL(of_device_uevent_modalias);
>
> diff --git a/drivers/of/module.c b/drivers/of/module.c
> index 1e735fc130ad..80879d2abea8 100644
> --- a/drivers/of/module.c
> +++ b/drivers/of/module.c
> @@ -8,21 +8,14 @@
> #include <linux/slab.h>
> #include <linux/string.h>
>
> -ssize_t of_modalias(const struct device_node *np, char *str, ssize_t len)
> +/* Do not use directly, use %pOFm format specifier instead */
> +size_t of_modalias(const struct device_node *np, char *str, size_t len)
> {
> const char *compat;
> char *c;
> struct property *p;
> - ssize_t csize;
> - ssize_t tsize;
> -
> - /*
> - * Prevent a kernel oops in vsnprintf() -- it only allows passing a
> - * NULL ptr when the length is also 0. Also filter out the negative
> - * lengths...
> - */
> - if ((len > 0 && !str) || len < 0)
> - return -EINVAL;
> + size_t csize;
> + size_t tsize;
>
> /* Name & Type */
> /* %p eats all alphanum characters, so %c must be used here */
> @@ -53,29 +46,15 @@ ssize_t of_modalias(const struct device_node *np, char *str, ssize_t len)
>
> int of_request_module(const struct device_node *np)
> {
> - char *str;
> - ssize_t size;
> - int ret;
> + char *str __free(kfree);
>
> if (!np)
> return -ENODEV;
>
> - size = of_modalias(np, NULL, 0);
> - if (size < 0)
> - return size;
> -
> - /* Reserve an additional byte for the trailing '\0' */
> - size++;
> -
> - str = kmalloc(size, GFP_KERNEL);
> + str = kasprintf(GFP_KERNEL, "%pOFm", np);
> if (!str)
> return -ENOMEM;
>
> - of_modalias(np, str, size);
> - str[size - 1] = '\0';
> - ret = request_module(str);
> - kfree(str);
> -
> - return ret;
> + return request_module(str);
> }
> EXPORT_SYMBOL_GPL(of_request_module);
> diff --git a/drivers/of/unittest.c b/drivers/of/unittest.c
> index daf9a2dddd7e..93921399f02d 100644
> --- a/drivers/of/unittest.c
> +++ b/drivers/of/unittest.c
> @@ -342,6 +342,8 @@ static void __init of_unittest_printf(void)
> of_unittest_printf_one(np, "%pOFc", "test-sub-device");
> of_unittest_printf_one(np, "%pOFC",
> "\"test-sub-device\",\"test-compat2\",\"test-compat3\"");
> + of_unittest_printf_one(np, "%pOFm",
> + "of:NdevT(null)Ctest-sub-deviceCtest-compat2Ctest-compat3");
> }
>
> struct node_hash {
> diff --git a/include/linux/of.h b/include/linux/of.h
> index f921786cb8ac..9fe7d17ce7e2 100644
> --- a/include/linux/of.h
> +++ b/include/linux/of.h
> @@ -382,7 +382,7 @@ extern int of_count_phandle_with_args(const struct device_node *np,
> const char *list_name, const char *cells_name);
>
> /* module functions */
> -extern ssize_t of_modalias(const struct device_node *np, char *str, ssize_t len);
> +extern size_t of_modalias(const struct device_node *np, char *str, size_t len);
> extern int of_request_module(const struct device_node *np);
>
> /* phandle iterator functions */
> @@ -762,10 +762,10 @@ static inline int of_count_phandle_with_args(const struct device_node *np,
> return -ENOSYS;
> }
>
> -static inline ssize_t of_modalias(const struct device_node *np, char *str,
> - ssize_t len)
> +static inline size_t of_modalias(const struct device_node *np, char *str,
> + size_t len)
> {
> - return -ENODEV;
> + return 0;
> }
>
> static inline int of_request_module(const struct device_node *np)
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 9d3dac38a3f4..6a4f99b39de0 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2169,10 +2169,10 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
>
> /* simple case without anything any more format specifiers */
> fmt++;
> - if (fmt[0] == '\0' || strcspn(fmt,"fnpPFcC") > 0)
> + if (fmt[0] == '\0' || strcspn(fmt,"fnpPFcCm") > 0)
> fmt = "f";
>
> - for (pass = false; strspn(fmt,"fnpPFcC"); fmt++, pass = true) {
> + for (pass = false; strspn(fmt,"fnpPFcCm"); fmt++, pass = true) {
> int precision;
> if (pass) {
> if (buf < end)
> @@ -2226,6 +2226,9 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
> has_mult = true;
> }
> break;
> + case 'm':
> + buf += of_modalias(dn, buf, end - buf);
> + break;
> default:
> break;
> }
> --
> 2.45.2
