[PATCH v1] core: Relax gpiod_chip_open() for symbolic links

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

User may ask device helper tool, for example, udev, to create a specific
symbolic link to a device node. GPIO chip character device node is not
exceptional. However, libgpiod in the commit d9b1c1f14c6b
("core: harden gpiod_chip_open()") went way too far in the hardening device
node check.

Relax that hardening for symbolic link to fix the regression.

Reproducer:

  % gpioinfo /dev/gpiochip5
  gpiochip5 - 16 lines:
      line   0:  "MUX33_DIR" "uart1-rx-oe" output active-high [used]
      ...

  % ln -sf /dev/gpiochip5 /dev/MyGPIO_5

  % gpioinfo /dev/MyGPIO_5
  gpioinfo: looking up chip /dev/MyGPIO_5: Inappropriate ioctl for device

Link: https://stackoverflow.com/questions/60057494/gpio-issue-with-sym-link
Fixes: d9b1c1f14c6b ("core: harden gpiod_chip_open()")
Cc: Bartosz Golaszewski <bartekgola@xxxxxxxxx>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---
 lib/core.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/lib/core.c b/lib/core.c
index 8352e18..32476ee 100644
--- a/lib/core.c
+++ b/lib/core.c
@@ -8,6 +8,7 @@
 /* Low-level, core library code. */
 
 #include <errno.h>
+#include <limits.h>
 #include <fcntl.h>
 #include <gpiod.h>
 #include <linux/gpio.h>
@@ -75,7 +76,7 @@ struct gpiod_chip {
 
 static bool is_gpiochip_cdev(const char *path)
 {
-	char *name, *pathcpy, *sysfsp, sysfsdev[16], devstr[16];
+	char *name, *realname, *sysfsp, sysfsdev[16], devstr[16];
 	struct stat statbuf;
 	bool ret = false;
 	int rv, fd;
@@ -85,6 +86,21 @@ static bool is_gpiochip_cdev(const char *path)
 	if (rv)
 		goto out;
 
+	/*
+	 * Is it a symbolic link?
+	 * We have to resolve symbolic link before checking the rest.
+	 */
+	if (S_ISLNK(statbuf.st_mode))
+		realname = realpath(path, NULL);
+	else
+		realname = strdup(path);
+	if (realname == NULL)
+		goto out;
+
+	rv = stat(realname, &statbuf);
+	if (rv)
+		goto out_free_realname;
+
 	/* Is it a character device? */
 	if (!S_ISCHR(statbuf.st_mode)) {
 		/*
@@ -94,20 +110,16 @@ static bool is_gpiochip_cdev(const char *path)
 		 * libgpiod from before the introduction of this routine.
 		 */
 		errno = ENOTTY;
-		goto out;
+		goto out_free_realname;
 	}
 
 	/* Get the basename. */
-	pathcpy = strdup(path);
-	if (!pathcpy)
-		goto out;
-
-	name = basename(pathcpy);
+	name = basename(realname);
 
 	/* Do we have a corresponding sysfs attribute? */
 	rv = asprintf(&sysfsp, "/sys/bus/gpio/devices/%s/dev", name);
 	if (rv < 0)
-		goto out_free_pathcpy;
+		goto out_free_realname;
 
 	if (access(sysfsp, R_OK) != 0) {
 		/*
@@ -149,8 +161,8 @@ static bool is_gpiochip_cdev(const char *path)
 
 out_free_sysfsp:
 	free(sysfsp);
-out_free_pathcpy:
-	free(pathcpy);
+out_free_realname:
+	free(realname);
 out:
 	return ret;
 }
-- 
2.25.0