[[PATCH v2 1/2] libfsimage: replace deprecated readdir_r() with readdir()

[[PATCH v2 1/2] libfsimage: replace deprecated readdir_r() with readdir()

[Chris Patterson]

From: Chris Patterson <pattersonc@ainfosec.com>

Replace the usage of readdir_r() with readdir() to address a
compilation error under glibc due to the deprecation of readdir_r
for their next release (2.24) [1, 2].

--

From the GNU libc manual [3]:
"
 It is expected that future versions of POSIX will obsolete readdir_r and
 mandate the level of thread safety for readdir which is provided by the
 GNU C Library and other implementations today.
"

There is a filed bug in the Austin Group Defect Tracker [4]  in which 'dalias'
proposes (in comment 0001632) that:
"
   I would like to propose an alternate solution. For readdir, replace the text:
    "The readdir() function need not be thread-safe."
   with:
    "If multiple threads call the readdir() function with the same directory
    stream argument and without synchronization to preclude simultaneous
    access, then the behavior is undefined."

   With this change, the clunky readdir_r function is no longer needed or
   useful, and should probably be deprecated. As the only reasonable way
   to meet the implementation requirements for readdir is to have the dirent
   buffer in the DIR structure, this change should not require any change to
   existing implementations.
"

[1] https://sourceware.org/ml/libc-alpha/2016-02/msg00093.html
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=19056
[3] https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html
[4] http://austingroupbugs.net/view.php?id=696

--

v2:
- Additional detail in commit message
- Cleanup additional related (no longer used) code

Signed-off-by: Chris Patterson <pattersonc@ainfosec.com>
---
 tools/libfsimage/common/fsimage_plugin.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/tools/libfsimage/common/fsimage_plugin.c b/tools/libfsimage/common/fsimage_plugin.c
index 3fa06c7..5ab8d93 100644
--- a/tools/libfsimage/common/fsimage_plugin.c
+++ b/tools/libfsimage/common/fsimage_plugin.c
@@ -122,8 +122,7 @@ fail:
 static int load_plugins(void)
 {
 	const char *fsdir = getenv("FSIMAGE_FSDIR");
-	struct dirent *dp = NULL;
-	struct dirent *dpp;
+	struct dirent *de;
 	DIR *dir = NULL;
 	char *tmp = NULL;
 	size_t name_max;
@@ -139,22 +138,17 @@ static int load_plugins(void)
 	if ((tmp = malloc(name_max + 1)) == NULL)
 		goto fail;
 
-	if ((dp = malloc(sizeof (struct dirent) + name_max + 1)) == NULL)
-		goto fail;
-
 	if ((dir = opendir(fsdir)) == NULL)
 		goto fail;
 
-	bzero(dp, sizeof (struct dirent) + name_max + 1);
-
-	while (readdir_r(dir, dp, &dpp) == 0 && dpp != NULL) {
-		if (strcmp(dpp->d_name, ".") == 0)
+	while ((de = readdir(dir)) != NULL) {
+		if (strcmp(de->d_name, ".") == 0)
 			continue;
-		if (strcmp(dpp->d_name, "..") == 0)
+		if (strcmp(de->d_name, "..") == 0)
 			continue;
 
 		(void) snprintf(tmp, name_max, "%s/%s/fsimage.so", fsdir,
-			dpp->d_name);
+			de->d_name);
 
 		if (init_plugin(tmp) != 0)
 			goto fail;
@@ -167,7 +161,6 @@ fail:
 	if (dir != NULL)
 		(void) closedir(dir);
 	free(tmp);
-	free(dp);
 	errno = err;
 	return (ret);
 }
-- 
2.1.4


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[[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()

[Chris Patterson]

From: Chris Patterson <pattersonc@ainfosec.com>

Replace the usage of readdir_r() with readdir() to address a
compilation error under glibc due to the deprecation of readdir_r
for their next release (2.24) [1, 2].

Remove code specific to usage of readdir_r which is no longer required,
such as zalloc_dirent().

--

From the GNU libc manual [3]:
"
 It is expected that future versions of POSIX will obsolete readdir_r and
 mandate the level of thread safety for readdir which is provided by the
 GNU C Library and other implementations today.
"

There is a filed bug in the Austin Group Defect Tracker [4]  in which 'dalias'
proposes (in comment 0001632) that:
"
   I would like to propose an alternate solution. For readdir, replace the text:
    "The readdir() function need not be thread-safe."
   with:
    "If multiple threads call the readdir() function with the same directory
    stream argument and without synchronization to preclude simultaneous
    access, then the behavior is undefined."

   With this change, the clunky readdir_r function is no longer needed or
   useful, and should probably be deprecated. As the only reasonable way
   to meet the implementation requirements for readdir is to have the dirent
   buffer in the DIR structure, this change should not require any change to
   existing implementations.
"

[1] https://sourceware.org/ml/libc-alpha/2016-02/msg00093.html
[2] https://sourceware.org/bugzilla/show_bug.cgi?id=19056
[3] https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html
[4] http://austingroupbugs.net/view.php?id=696

--

v2:
- Additional detail in commit message
- Fix readdir loop logic (do not treat NULL as error)
- Cleanup additional related (no longer used) code

Signed-off-by: Chris Patterson <pattersonc@ainfosec.com>
---
 tools/libxl/libxl_pvusb.c | 42 +++---------------------------------------
 tools/libxl/libxl_utils.c | 14 +-------------
 2 files changed, 4 insertions(+), 52 deletions(-)

diff --git a/tools/libxl/libxl_pvusb.c b/tools/libxl/libxl_pvusb.c
index 9f1e842..01819bd 100644
--- a/tools/libxl/libxl_pvusb.c
+++ b/tools/libxl/libxl_pvusb.c
@@ -508,19 +508,10 @@ int libxl_devid_to_device_usbctrl(libxl_ctx *ctx,
     return rc;
 }
 
-static void *zalloc_dirent(libxl__gc *gc, const char *dirpath)
-{
-    size_t need = offsetof(struct dirent, d_name) +
-                  pathconf(dirpath, _PC_NAME_MAX) + 1;
-
-    return libxl__zalloc(gc, need);
-}
-
 static char *usbdev_busaddr_to_busid(libxl__gc *gc, int bus, int addr)
 {
     DIR *dir;
     char *busid = NULL;
-    struct dirent *de_buf;
     struct dirent *de;
 
     /* invalid hostbus or hostaddr */
@@ -533,22 +524,12 @@ static char *usbdev_busaddr_to_busid(libxl__gc *gc, int bus, int addr)
         return NULL;
     }
 
-    de_buf = zalloc_dirent(gc, SYSFS_USB_DEV);
-
-    for (;;) {
+    while ((de = readdir(dir)) != NULL) {
         char *filename;
         void *buf;
         int busnum = -1;
         int devnum = -1;
 
-        int r = readdir_r(dir, de_buf, &de);
-        if (r) {
-            LOGE(ERROR, "failed to readdir %s", SYSFS_USB_DEV);
-            break;
-        }
-        if (!de)
-            break;
-
         if (!strcmp(de->d_name, ".") ||
             !strcmp(de->d_name, ".."))
             continue;
@@ -1157,9 +1138,7 @@ static int usbdev_get_all_interfaces(libxl__gc *gc, const char *busid,
 {
     DIR *dir;
     char *buf;
-    struct dirent *de_buf;
     struct dirent *de;
-    int rc;
 
     *intfs = NULL;
     *num = 0;
@@ -1172,19 +1151,7 @@ static int usbdev_get_all_interfaces(libxl__gc *gc, const char *busid,
         return ERROR_FAIL;
     }
 
-    de_buf = zalloc_dirent(gc, SYSFS_USB_DEV);
-
-    for (;;) {
-        int r = readdir_r(dir, de_buf, &de);
-
-        if (r) {
-            LOGE(ERROR, "failed to readdir %s", SYSFS_USB_DEV);
-            rc = ERROR_FAIL;
-            goto out;
-        }
-        if (!de)
-            break;
-
+    while ((de = readdir(dir)) != NULL) {
         if (!strcmp(de->d_name, ".") ||
             !strcmp(de->d_name, ".."))
             continue;
@@ -1196,11 +1163,8 @@ static int usbdev_get_all_interfaces(libxl__gc *gc, const char *busid,
         }
     }
 
-    rc = 0;
-
-out:
     closedir(dir);
-    return rc;
+    return 0;
 }
 
 /* Encode usb interface so that it could be written to xenstore as a key.
diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index ceb8825..5730774 100644
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -548,21 +548,9 @@ int libxl__remove_directory(libxl__gc *gc, const char *dirpath)
         goto out;
     }
 
-    size_t need = offsetof(struct dirent, d_name) +
-        pathconf(dirpath, _PC_NAME_MAX) + 1;
-    struct dirent *de_buf = libxl__zalloc(gc, need);
     struct dirent *de;
 
-    for (;;) {
-        int r = readdir_r(d, de_buf, &de);
-        if (r) {
-            LOGE(ERROR, "failed to readdir %s for removal", dirpath);
-            rc = ERROR_FAIL;
-            break;
-        }
-        if (!de)
-            break;
-
+    while ((de = readdir(d)) != NULL) {
         if (!strcmp(de->d_name, ".") ||
             !strcmp(de->d_name, ".."))
             continue;
-- 
2.1.4


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Re: [[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()

[Ian Jackson]

Chris Patterson writes ("[[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()"):
> -    for (;;) {
> +    while ((de = readdir(dir)) != NULL) {
...
> -        int r = readdir_r(dir, de_buf, &de);
> -        if (r) {
> -            LOGE(ERROR, "failed to readdir %s", SYSFS_USB_DEV);
> -            break;

Sadly this is not right because it mishandles errors when reading the
directory, treating them all as EOF.  See the error handling info
in the specification for readdir:
  http://pubs.opengroup.org/onlinepubs/9699919799/functions/readdir.html

Ian.

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Re: [[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()

[Chris Patterson]

On Thu, Jun 2, 2016 at 6:11 AM, Ian Jackson <Ian.Jackson@eu.citrix.com> wrote:
> Chris Patterson writes ("[[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()"):
>> -    for (;;) {
>> +    while ((de = readdir(dir)) != NULL) {
> ...
>> -        int r = readdir_r(dir, de_buf, &de);
>> -        if (r) {
>> -            LOGE(ERROR, "failed to readdir %s", SYSFS_USB_DEV);
>> -            break;
>
> Sadly this is not right because it mishandles errors when reading the
> directory, treating them all as EOF.  See the error handling info
> in the specification for readdir:
>   http://pubs.opengroup.org/onlinepubs/9699919799/functions/readdir.html
>

You're right, it should check for the error afterwards.

How about something along the lines of:

int saved_errno = errno;
errno = 0;
while ((de = readdir(dir)) != NULL) {
    ...
}

if (errno) {
   LOGE(ERROR, "readdir failed: %s", strerror(errno));
   rc = ERROR_FAIL;
}
errno = saved_errno;
...

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Re: [[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()

[Ian Jackson]

Chris Patterson writes ("Re: [[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()"):
> You're right, it should check for the error afterwards.
> 
> How about something along the lines of:
> 
> int saved_errno = errno;
> errno = 0;
> while ((de = readdir(dir)) != NULL) {
>     ...

Wrong because you need to set errno=0 before each call to readdir.
I really think you should abandon your efforts to keep the readdir
call inside the while() condition :-).

> if (errno) {
>    LOGE(ERROR, "readdir failed: %s", strerror(errno));
>    rc = ERROR_FAIL;
> }
> errno = saved_errno;

I haven't eyeballed the context in detail but I don't understand why
you think it necessary to save and restore errno.  All the many system
and library calls made throughout this code may overwrite it anyway.

Ian.

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Re: [[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()

[Chris Patterson]

On Thu, Jun 2, 2016 at 12:13 PM, Ian Jackson <Ian.Jackson@eu.citrix.com> wrote:
> Chris Patterson writes ("Re: [[PATCH v2 2/2] libxl: replace deprecated readdir_r() with readdir()"):
>> You're right, it should check for the error afterwards.
>>
>> How about something along the lines of:
>>
>> int saved_errno = errno;
>> errno = 0;
>> while ((de = readdir(dir)) != NULL) {
>>     ...
>
> Wrong because you need to set errno=0 before each call to readdir.
> I really think you should abandon your efforts to keep the readdir
> call inside the while() condition :-).
>

I agree.  How does something like this look?

-        int r = readdir_r(dir, de_buf, &de);
-
-        if (r) {
+       errno = 0;
+       de = readdir(dir);
+
+       if (!de && errno) {

And I'll apply the same construct for tools/libfsimage/common/fsimage_plugin.c.

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