X-Git-Url: http://drtracing.org/?a=blobdiff_plain;f=src%2Frseq.c;h=ae72eaa22f556b64a0d763eaa179e9e4960c0162;hb=c0de0012017aff7b1f310b9e34bac2d842d46a2b;hp=e904b443e1212fc63b95da1ada172e50db241612;hpb=baa98a34eddcc4a09895aad6edb1af483e4164b7;p=librseq.git

diff --git a/src/rseq.c b/src/rseq.c
index e904b44..ae72eaa 100644
--- a/src/rseq.c
+++ b/src/rseq.c
@@ -21,6 +21,7 @@
 #include <linux/auxvec.h>
 
 #include <rseq/rseq.h>
+#include "smp.h"
 
 #ifndef AT_RSEQ_FEATURE_SIZE
 # define AT_RSEQ_FEATURE_SIZE		27
@@ -70,6 +71,12 @@ static int rseq_reg_success;	/* At least one rseq registration has succeded. */
 /* Original struct rseq allocation size is 32 bytes. */
 #define ORIG_RSEQ_ALLOC_SIZE		32
 
+/*
+ * The alignment on RSEQ_THREAD_AREA_ALLOC_SIZE guarantees that the
+ * rseq_abi structure allocated size is at least
+ * RSEQ_THREAD_AREA_ALLOC_SIZE bytes to hold extra space for yet unknown
+ * kernel rseq extensions.
+ */
 static
 __thread struct rseq_abi __rseq_abi __attribute__((tls_model("initial-exec"), aligned(RSEQ_THREAD_AREA_ALLOC_SIZE))) = {
 	.cpu_id = RSEQ_ABI_CPU_ID_UNINITIALIZED,
@@ -167,16 +174,31 @@ unsigned int get_rseq_feature_size(void)
 		return ORIG_RSEQ_FEATURE_SIZE;
 }
 
+/*
+ * Initialize the public symbols for the rseq offset, size, feature size and
+ * flags prior to registering threads. If glibc owns the registration, get the
+ * values from its public symbols.
+ */
 static
 void rseq_init(void)
 {
+	/* Ensure initialization is only done once. */
 	if (RSEQ_READ_ONCE(init_done))
 		return;
 
+	/*
+	 * Take the mutex, check the initialization flag again and atomically
+	 * set it to ensure we are the only thread doing the initialization.
+	 */
 	pthread_mutex_lock(&init_lock);
 	if (init_done)
 		goto unlock;
 	RSEQ_WRITE_ONCE(init_done, 1);
+
+	/*
+	 * Check for glibc rseq support, if the 3 public symbols are found and
+	 * the rseq_size is not zero, glibc owns the registration.
+	 */
 	libc_rseq_offset_p = dlsym(RTLD_NEXT, "__rseq_offset");
 	libc_rseq_size_p = dlsym(RTLD_NEXT, "__rseq_size");
 	libc_rseq_flags_p = dlsym(RTLD_NEXT, "__rseq_flags");
@@ -187,18 +209,42 @@ void rseq_init(void)
 		rseq_size = *libc_rseq_size_p;
 		rseq_flags = *libc_rseq_flags_p;
 		rseq_feature_size = get_rseq_feature_size();
+
+		/*
+		 * The registered rseq area could be smaller than the feature
+		 * size reported by the kernel auxval. Cap it to the rseq size
+		 * so we don't try to access features past the end of the rseq
+		 * area.
+		 */
 		if (rseq_feature_size > rseq_size)
 			rseq_feature_size = rseq_size;
 		goto unlock;
 	}
+
+	/* librseq owns the registration */
 	rseq_ownership = 1;
+
+	/* Calculate the offset of the rseq area from the thread pointer. */
+	rseq_offset = (uintptr_t)&__rseq_abi - (uintptr_t)rseq_thread_pointer();
+
+	/* rseq flags are deprecated, always set to 0. */
+	rseq_flags = 0;
+
+	/*
+	 * Check if the rseq syscall is available, if not set the size and
+	 * feature_size to 0.
+	 */
 	if (!rseq_available(RSEQ_AVAILABLE_QUERY_KERNEL)) {
 		rseq_size = 0;
 		rseq_feature_size = 0;
 		goto unlock;
 	}
-	rseq_offset = (uintptr_t)&__rseq_abi - (uintptr_t)rseq_thread_pointer();
-	rseq_flags = 0;
+
+	/*
+	 * If the feature size matches the original ABI (20), set the size to
+	 * match the original ABI allocation (32), otherwise use the allocated
+	 * size.
+	 */
 	rseq_feature_size = get_rseq_feature_size();
 	if (rseq_feature_size == ORIG_RSEQ_FEATURE_SIZE)
 		rseq_size = ORIG_RSEQ_ALLOC_SIZE;
@@ -243,3 +289,8 @@ int32_t rseq_fallback_current_node(void)
 	}
 	return (int32_t) node_id;
 }
+
+int rseq_get_max_nr_cpus(void)
+{
+	return get_possible_cpus_array_len();
+}