Message-ID: <20191218112659.crabhqkbcnxd6fo6@pali>
Date:   Wed, 18 Dec 2019 12:26:59 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Abhishek Pandit-Subedi <abhishekpandit@...omium.org>,
        linux-input@...r.kernel.org,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Logan Gunthorpe <logang@...tatee.com>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        Kirill Smelkov <kirr@...edi.com>
Subject: Re: [PATCH] Input: uinput - Add UI_SET_UNIQ ioctl handler

On Wednesday 18 December 2019 12:02:24 Pali Rohár wrote:
> On Friday 06 December 2019 09:40:48 Dmitry Torokhov wrote:
> > On Fri, Dec 06, 2019 at 10:11:14AM +0100, Pali Rohár wrote:
> > > On Thursday 05 December 2019 12:03:05 Abhishek Pandit-Subedi wrote:
> > > > On Thu, Dec 5, 2019 at 2:52 AM Pali Rohár <pali.rohar@...il.com> wrote:
> > > > >
> > > > > On Tuesday 03 December 2019 11:11:12 Dmitry Torokhov wrote:
> > > > > > On Tue, Dec 03, 2019 at 06:38:21PM +0100, Pali Rohár wrote:
> > > > > > > On Tuesday 03 December 2019 00:09:47 Pali Rohár wrote:
> > > > > > >
> > > > > > > Hi Dmitry!
> > > > > > >
> > > > > > > I was looking again at those _IOW defines for ioctl calls and I have
> > > > > > > another argument why not specify 'char *' in _IOW:
> > > > > > >
> > > > > > > All ioctls in _IOW() specify as a third macro argument type which is
> > > > > > > passed as pointer to the third argument for ioctl() syscall.
> > > > > > >
> > > > > > > So e.g.:
> > > > > > >
> > > > > > >   #define EVIOCSCLOCKID _IOW('E', 0xa0, int)
> > > > > > >
> > > > > > > is called from userspace as:
> > > > > > >
> > > > > > >   int val;
> > > > > > >   ioctl(fd, EVIOCSCLOCKID, &val);
> > > > > > >
> > > > > > > Or
> > > > > > >
> > > > > > >   #define EVIOCSMASK _IOW('E', 0x93, struct input_mask)
> > > > > > >
> > > > > > > is called as:
> > > > > > >
> > > > > > >   struct input_mask val;
> > > > > > >   ioctl(fd, EVIOCSMASK, &val);
> > > > > > >
> > > > > > > So basically third argument for _IOW specify size of byte buffer passed
> > > > > > > as third argument for ioctl(). In _IOW is not specified pointer to
> > > > > > > struct input_mask, but struct input_mask itself.
> > > > > > >
> > > > > > > And in case you define
> > > > > > >
> > > > > > >   #define MY_NEW_IOCTL _IOW(UINPUT_IOCTL_BASE, 200, char*)
> > > > > > >
> > > > > > > then you by above usage you should pass data as:
> > > > > > >
> > > > > > >   char *val = "DATA";
> > > > > > >   ioctl(fd, MY_NEW_IOCTL, &val);
> > > > > > >
> > > > > > > Which is not same as just:
> > > > > > >
> > > > > > >   ioctl(fd, MY_NEW_IOCTL, "DATA");
> > > > > > >
> > > > > > > As in former case you passed pointer to pointer to data and in later
> > > > > > > case you passed only pointer to data.
> > > > > > >
> > > > > > > It just mean that UI_SET_PHYS is already defined inconsistently which is
> > > > > > > also reason why compat ioctl for it was introduced.
> > > > > >
> > > > > > Yes, you are right. UI_SET_PHYS is messed up. I guess the question is
> > > > > > what to do with all of this...
> > > > > >
> > > > > > Maybe we should define
> > > > > >
> > > > > > #define UI_SET_PHYS_STR(len)  _IOC(_IOC_WRITE, UINPUT_IOCTL_BASE, 111, len)
> > > > > > #define UI_SET_UNIQ_STR(len)  _IOC(_IOC_WRITE, UINPUT_IOCTL_BASE, 112, len)
> > > > >
> > > > > I'm not sure if this is ideal. Normally in C strings are nul-termined,
> > > > > so functions/macros do not take buffer length.
> > > > Except strncpy, strndup, snprintf, etc. all expect a buffer length. At
> > > 
> > > This is something different as for these functions you pass buffer and
> > > length of buffer which is used in write mode -- not for read mode.
> > > 
> > > > the user to kernel boundary of ioctl, I think we should require size
> > > > of the user buffer regardless of the data type.
> > > > 
> > > > > _STR therefore in names looks inconsistent.
> > > > The _STR suffix is odd (what to name UI_SET_PHYS_STR then??) but
> > > > requiring the length seems to be common across various ioctls.
> > > > * input.h requires a length when requesting the phys and uniq
> > > > (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/input.h#n138)
> > > > * Same with HIDRAW when setting and getting features:
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/hidraw.h#n40,
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/hidraw/hid-example.c#n88
> > > 
> > > All these ioctls where is passed length are in opposite direction
> > > (_IOC_READ) as our PHYS and UNIQ (_IOC_WRITE).
> > > 
> > > I fully agree that when you need to read something from kernel
> > > (_IOC_READ) and then writing it to userspace, you need to specify length
> > > of userspace buffer. Exactly same as with userspace functions like
> > > memcpy, snprintf, etc... as you pointed. Otherwise you get buffer
> > > overflow as callee does not know length of buffer.
> > > 
> > > But here we we have there quite different problem, we need to write
> > > something to kernel from userspace (_IOC_WRITE) and we are passing
> > > nul-term string. So in this case specifying size is not required as it
> > > is implicitly specified as part of passed string.
> > 
> > With the new IOCTL definitions it does not need to be a NULL-terminated
> > string. It can be a buffer of characters with given length, and kernel
> > will NULL-terminate as this it what it wants, not what the caller has to
> > give.
> 
> Hi Dmitry! I was thinking more about this problem and I will propose
> alternative solution, but first with details...
> 
> I think that we should use NULL terminated strings. Or better disallow
> NULL byte inside string. Reason: all userspace application expects that
> input device name would be NULL terminated which implies that in the
> middle of name cannot be NULL byte.
> 
> So this must apply also for new PHYS / UNIQ ioctl API. If we want in our
> ioctl API to use buffer + size (with upper bound limit for size) instead
> of passing NULL term string (with upper bound limit for string size)
> then kernel have to add a leading NULL byte to string, plus check that
> in the buffer there is no NULL byte. I guess this a very little
> complicate code, but nothing which is problematic.
> 
> And on the userspace part. Now when userspace want to pass constant
> string for device name, it just call
> 
>   ioctl(fd, UI_SET_PHYS, "my name of device");
> 
> After adding a new ioctl with buffer + size API, userspace would have to
> call:
> 
>   ioctl(fd, UI_SET_PHYS_STR(strlen("my name of device")), "my name of device");
> 
> which looks strange, so programmers would had to move device name into
> new variable:
> 
>   const char *name = "my name of device";
>   ioctl(fd, UI_SET_PHYS_STR(strlen(name)), name);
> 
> For me the old ioctl API looks easier to use (no need for strlen() or
> extra variable), but this is just my preference of usage -- as it is
> simpler for me. Maybe you would have different opinion...
> 
> And now question: Why we have uinput_compat_ioctl()? It is there only
> because size part of IOCTL number is different on 32bit and 64bit
> systems. As we know size part of UI_SET_PHYS is wrong and does not make
> sense...
> 
> Would not it be better to change size of UI_SET_PHYS to just zero and
> then when matching ioctl number just ignore size for this UI_SET_PHYS
> ioctl? Same for UI_BEGIN_FF_UPLOAD_COMPAT and UI_END_FF_UPLOAD_COMPAT
> added in: https://git.kernel.org/tip/tip/c/7c7da40
> 
> And we would not have to deal with uinput_compat_ioctl() at all.

Below is example how change for removing UI_SET_PHYS_COMPAT may look
like. As header file is not changed and UI_SET_PHYS accepts any size
argument, it therefore accept also 32bit and 64bit integer. So no
existing 32bit applications which use UI_SET_PHYS on 64bit kernel would
not be broken...

Is not this better change then introducing a new UI_SET_PHYS_STR ioctl
number? Because introduction of new IOCTL number has one big
disadvantage: Userspace applications needs to support fallback to old
number as older versions of kernels would be in use for a long time. And
because kernel does not have to remove old IOCTL number for backward
compatibility there is basically no need for userspace application to
user new UI_SET_PHYS_STR IOCTL number...

diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
index fd253781b..b645210d5 100644
--- a/drivers/input/misc/uinput.c
+++ b/drivers/input/misc/uinput.c
@@ -915,22 +915,6 @@ static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 		retval = uinput_set_bit(arg, propbit, INPUT_PROP_MAX);
 		goto out;
 
-	case UI_SET_PHYS:
-		if (udev->state == UIST_CREATED) {
-			retval = -EINVAL;
-			goto out;
-		}
-
-		phys = strndup_user(p, 1024);
-		if (IS_ERR(phys)) {
-			retval = PTR_ERR(phys);
-			goto out;
-		}
-
-		kfree(udev->dev->phys);
-		udev->dev->phys = phys;
-		goto out;
-
 	case UI_BEGIN_FF_UPLOAD:
 		retval = uinput_ff_upload_from_user(p, &ff_up);
 		if (retval)
@@ -1023,6 +1007,22 @@ static long uinput_ioctl_handler(struct file *file, unsigned int cmd,
 	case UI_ABS_SETUP & ~IOCSIZE_MASK:
 		retval = uinput_abs_setup(udev, p, size);
 		goto out;
+
+	case UI_SET_PHYS & ~IOCSIZE_MASK:
+		if (udev->state == UIST_CREATED) {
+			retval = -EINVAL;
+			goto out;
+		}
+
+		phys = strndup_user(p, 1024);
+		if (IS_ERR(phys)) {
+			retval = PTR_ERR(phys);
+			goto out;
+		}
+
+		kfree(udev->dev->phys);
+		udev->dev->phys = phys;
+		goto out;
 	}
 
 	retval = -EINVAL;
@@ -1042,8 +1042,6 @@ static long uinput_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
  * These IOCTLs change their size and thus their numbers between
  * 32 and 64 bits.
  */
-#define UI_SET_PHYS_COMPAT		\
-	_IOW(UINPUT_IOCTL_BASE, 108, compat_uptr_t)
 #define UI_BEGIN_FF_UPLOAD_COMPAT	\
 	_IOWR(UINPUT_IOCTL_BASE, 200, struct uinput_ff_upload_compat)
 #define UI_END_FF_UPLOAD_COMPAT		\
@@ -1053,9 +1051,6 @@ static long uinput_compat_ioctl(struct file *file,
 				unsigned int cmd, unsigned long arg)
 {
 	switch (cmd) {
-	case UI_SET_PHYS_COMPAT:
-		cmd = UI_SET_PHYS;
-		break;
 	case UI_BEGIN_FF_UPLOAD_COMPAT:
 		cmd = UI_BEGIN_FF_UPLOAD;
 		break;
diff --git a/include/uapi/linux/uinput.h b/include/uapi/linux/uinput.h
index c9e677e3a..6bda2a142 100644
--- a/include/uapi/linux/uinput.h
+++ b/include/uapi/linux/uinput.h
@@ -142,6 +142,8 @@ struct uinput_abs_setup {
 #define UI_SET_LEDBIT		_IOW(UINPUT_IOCTL_BASE, 105, int)
 #define UI_SET_SNDBIT		_IOW(UINPUT_IOCTL_BASE, 106, int)
 #define UI_SET_FFBIT		_IOW(UINPUT_IOCTL_BASE, 107, int)
+/* Argument is nul-term string and for backward compatibility is there
+ * specified char*, but size argument (char *) is ignored by this ioctl */
 #define UI_SET_PHYS		_IOW(UINPUT_IOCTL_BASE, 108, char*)
 #define UI_SET_SWBIT		_IOW(UINPUT_IOCTL_BASE, 109, int)
 #define UI_SET_PROPBIT		_IOW(UINPUT_IOCTL_BASE, 110, int)


-- 
Pali Rohár
pali.rohar@...il.com

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