Implement libcap:cap_proc_root() function.

This is needed to locally configure libcap to find the pid data
if the proc filesystem is not mounted at "/proc" (rare). Currently
libcap only uses this info to implement cap_iab_get_pid().

This brings libcap back to parity with the Go "cap" package.

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>

4 files changed, 74 insertions, 4 deletions

diff --git a/libcap/cap_alloc.c b/libcap/cap_alloc.c
index df1a275..5733e2f 100644
--- a/libcap/cap_alloc.c
+++ b/libcap/cap_alloc.c
@@ -8,19 +8,22 @@
 #include "libcap.h"
 
 /*
- * This gets set via the pre-main() executed constructor function below it.
+ * These get set via the pre-main() executed constructor function below it.
  */
 static cap_value_t _cap_max_bits;
 
-__attribute__((constructor (300))) static void _initialize_libcap(void) {
+__attribute__((constructor (300))) static void _initialize_libcap(void)
+{
     if (_cap_max_bits) {
 	return;
     }
     cap_set_syscall(NULL, NULL);
     _binary_search(_cap_max_bits, cap_get_bound, 0, __CAP_MAXBITS, __CAP_BITS);
+    cap_proc_root("/proc");
 }
 
-cap_value_t cap_max_bits(void) {
+cap_value_t cap_max_bits(void)
+{
     return _cap_max_bits;
 }
 
diff --git a/libcap/cap_test.c b/libcap/cap_test.c
index 41192b7..9fa7300 100644
--- a/libcap/cap_test.c
+++ b/libcap/cap_test.c
@@ -118,6 +118,7 @@ static int test_alloc(void)
     cap_t c;
     cap_iab_t iab;
     cap_launch_t launcher;
+    char *old_root;
 
     c = cap_init();
     if (c == NULL) {
@@ -153,6 +154,31 @@ static int test_alloc(void)
 	goto drop_launcher;
     }
 
+    old_root = cap_proc_root("blah");
+    if (old_root == NULL || strcmp(old_root, "/proc") != 0) {
+	printf("bad initial proc_root [%s]\n", old_root);
+	retval = -1;
+    }
+    if (cap_free(old_root)) {
+	perror("unable to free old proc root");
+	retval = -1;
+    }
+    if (retval) {
+	goto drop_launcher;
+    }
+    old_root = cap_proc_root("/proc");
+    if (strcmp(old_root, "blah") != 0) {
+	printf("bad proc_root value [%s]\n", old_root);
+	retval = -1;
+    }
+    if (cap_free(old_root)) {
+	perror("unable to free replacement proc root");
+	retval = -1;
+    }
+    if (retval) {
+	goto drop_launcher;
+    }
+
 drop_launcher:
     if (cap_free(launcher)) {
 	perror("failed to free launcher");
diff --git a/libcap/cap_text.c b/libcap/cap_text.c
index a0857bc..013eb1e 100644
--- a/libcap/cap_text.c
+++ b/libcap/cap_text.c
@@ -667,6 +667,31 @@ static __u32 _parse_vec_string(__u32 *vals, const char *c, int invert)
     return ~0;
 }
 
+/*
+ * libcap believes this is the root of the mounted "/proc"
+ * filesystem
+ */
+static char *_cap_proc_dir;
+
+/*
+ * cap_proc_root reads and (optionally: when root != NULL) changes
+ * libcap's notion of where the "/proc" filesystem is mounted. It
+ * defaults to the value "/proc". Note, this is a global value and not
+ * considered thread safe to write - so the client should take
+ * suitable care when changing it. Further, libcap will allocate
+ * memory for storing the replacement root, and it is this memory that
+ * is returned. So, when changing the value, the caller should
+ * cap_free(the-return-value) when done with it.
+ */
+char *cap_proc_root(const char *root)
+{
+    char *old = _cap_proc_dir;
+    if (root != NULL) {
+	_cap_proc_dir = _libcap_strdup(root);
+    }
+    return old;
+}
+
 #define PROC_LINE_MAX (8 + 8*_LIBCAP_CAPABILITY_U32S + 100)
 /*
  * cap_iab_get_pid fills an IAB tuple from the content of
@@ -680,7 +705,7 @@ cap_iab_t cap_iab_get_pid(pid_t pid)
     FILE *file;
     char line[PROC_LINE_MAX];
 
-    if (asprintf(&path, "/proc/%d/status", pid) <= 0) {
+    if (asprintf(&path, "%s/%d/status", _cap_proc_dir, pid) <= 0) {
 	return NULL;
     }
     file = fopen(path, "r");
diff --git a/libcap/include/sys/capability.h b/libcap/include/sys/capability.h
index f98da5a..59f9377 100644
--- a/libcap/include/sys/capability.h
+++ b/libcap/include/sys/capability.h
@@ -54,6 +54,22 @@ typedef int cap_value_t;
 extern cap_value_t cap_max_bits(void);
 
 /*
+ * cap_proc_root reads and (optionally: when root != NULL) changes
+ * libcap's notion of where the "/proc" filesystem is mounted. It
+ * defaults to the value "/proc".
+ *
+ * Note, this is a global value and not considered thread safe to
+ * write - so the client should take suitable care when changing
+ * it.
+ *
+ * Further, libcap will allocate a memory copy for storing the
+ * replacement root, and it is this kind of memory that is returned.
+ * So, when changing the value, the caller should
+ * cap_free(the-return-value) when done with it.
+ */
+extern char *cap_proc_root(const char *root);
+
+/*
  * Set identifiers
  */
 typedef enum {