Add TLS (thread-local storage) tutorial

tls/.gitignore

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+/__thread
+/__thread-static
+/tsd
+/tsd-static

tls/Makefile

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+CFLAGS = -Wall
+LDLIBS = -lpthread
+
+.PHONY: all clean
+
+all: __thread __thread-static tsd tsd-static indirect_modify
+
+__thread-static: __thread.o
+	$(CC) $(LDLFAGS) -static -o $@ $^ $(LDLIBS)
+
+__thread: __thread.o
+
+__thread.o: __thread.c
+
+tsd-static: tsd.o
+	$(CC) $(LDLFAGS) -static -o $@ $^ $(LDLIBS)
+
+tsd: tsd.o
+
+tsd.o: tsd.c
+
+indirect_modify: indirect_modify.o
+
+indirect_modify.o: indirect_modify.c
+	$(CC) $(CFLAGS) -fstack-protector-strong -c -o $@ $<
+
+clean:
+	-rm -f __thread.o __thread __thread-static
+	-rm -f tsd.o tsd tsd-static
+	-rm -f indirect_modify.o indirect_modify
+	-rm -f *~

tls/README.md

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+# Thread-Local Storage
+
+This is a tutorial / presentation on thread-local storage (TLS).
+TLS is a per-thread memory area used to store thread-specific data.
+Global data that are required to be specific per thread are usually stored as part of the TLS.
+
+https://docs.oracle.com/cd/E19120-01/open.solaris/819-0690/chapter8-1/index.html
+
+https://chao-tic.github.io/blog/2018/12/25/tls
+
+https://wiki.osdev.org/Thread_Local_Storage

tls/__thread.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/types.h>
+
+static __thread size_t me;
+static __thread size_t you;
+static __thread size_t them = 543;
+
+static void *tf(void *arg)
+{
+	unsigned long id = (unsigned long) arg;
+	static __thread size_t us = 13;
+
+	me = 200 + id;
+	us += id*id;
+	printf("me in thread (%p): %zu\n", &me, me);
+	printf("you in thread (%p): %zu\n", &you, you);
+	printf("you in thread (%p): %zu\n", &them, them);
+	printf("us in thread (%p): %zu\n", &us, us);
+	printf("errno in thread (%p): %d\n", &errno, errno);
+
+	return NULL;
+}
+
+int main(void)
+{
+	pthread_t new;
+	int err;
+
+	me = 100;
+	you = 999;
+	them = 3;
+	printf("me before create (%p): %zu\n", &me, me);
+	printf("you before create (%p): %zu\n", &you, you);
+	printf("them before create (%p): %zu\n", &them, them);
+	printf("errno before create (%p): %d\n", &errno, errno);
+
+	err = pthread_create(&new, NULL, tf, (void *) 1);
+	if (err < 0) {
+		perror("pthread_create");
+		exit(EXIT_FAILURE);
+	}
+	err = pthread_create(&new, NULL, tf, (void *) 2);
+	if (err < 0) {
+		perror("pthread_create");
+		exit(EXIT_FAILURE);
+	}
+
+	printf("me after create before join (%p): %zu\n", &me, me);
+	printf("you after create before join (%p): %zu\n", &you, you);
+	printf("them after create before join (%p): %zu\n", &them, them);
+	printf("errno after create before join (%p): %d\n", &errno, errno);
+	pthread_join(new, NULL);
+	printf("me after join (%p): %zu\n", &me, me);
+	printf("you after join (%p): %zu\n", &you, you);
+	printf("them after join (%p): %zu\n", &them, them);
+	printf("errno after join (%p): %d\n", &errno, errno);
+
+	return 0;
+}

tls/indirect_modify.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <sys/types.h>
+
+/*
+ * Offset is determined by running the program and computing
+ * (&age - &placeholder) / sizeof(unsigned int).
+ */
+#define OFFSET_FROM_LOCAL_TO_TLS	522
+
+/*
+ * &age is at fs:0xfffffffffffffffc, i.e. tcb-0x4
+ * stack guard is at fs:0x28, i.e. tcb+x028
+ * offset is (0x28-(-0x04))/4 = 11
+ */
+#define OFFSET_FROM_TLS_TO_STACK_GUARD	11
+
+static __thread unsigned int age;
+
+static void *tf(void *arg)
+{
+	unsigned int placeholder;
+	unsigned int *ptr;
+	unsigned long *stack_guard_ptr;
+
+	ptr = &placeholder;
+	printf("placeholder address: %p\n", &placeholder);
+
+	age = 22;
+	printf("age (address: %p): %u\n", &age, age);
+
+	* (ptr + OFFSET_FROM_LOCAL_TO_TLS) = 99;
+	printf("age (address: %p): %u\n", &age, age);
+
+	/* Print then overwrite stack guard with some gibberish. */
+	stack_guard_ptr = (unsigned long *) (ptr + OFFSET_FROM_LOCAL_TO_TLS + OFFSET_FROM_TLS_TO_STACK_GUARD);
+	printf("stack guard / canary (address: %p): 0x%016lx\n", stack_guard_ptr, *stack_guard_ptr);
+	* (stack_guard_ptr) = 0xaabbccddeeff0011;
+
+	return NULL;
+}
+
+int main(void)
+{
+	pthread_t new;
+	int err;
+
+	err = pthread_create(&new, NULL, tf, (void *) 1);
+	if (err < 0) {
+		perror("pthread_create");
+		exit(EXIT_FAILURE);
+	}
+	pthread_join(new, NULL);
+
+	return 0;
+}

tls/tsd.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <sys/types.h>
+
+static pthread_key_t me_key;
+static pthread_key_t you_key;
+static pthread_key_t them_key;
+
+static void *tf(void *arg)
+{
+	pthread_setspecific(me_key, (const void *) 200);
+	printf("me_key in thread (%p): %u, me value in thread: %zu\n", &me_key, me_key, (size_t) pthread_getspecific(me_key));
+	printf("you_key in thread (%p): %u, you value in thread: %zu\n", &you_key, you_key, (size_t) pthread_getspecific(you_key));
+	printf("them_key in thread (%p): %u, them value in thread: %zu\n", &them_key, them_key, (size_t) pthread_getspecific(them_key));
+
+	return NULL;
+}
+
+int main(void)
+{
+	pthread_t new;
+	int err;
+
+	pthread_key_create(&me_key, NULL);
+	pthread_key_create(&you_key, NULL);
+	pthread_key_create(&them_key, NULL);
+
+	pthread_setspecific(me_key, (const void *) 100);
+	pthread_setspecific(you_key, (const void *) 999);
+	pthread_setspecific(them_key, (const void *) 3);
+	printf("me_key before create (%p): %u, me value before create: %zu\n", &me_key, me_key, (size_t) pthread_getspecific(me_key));
+	printf("you_key before create (%p): %u, me value before create: %zu\n", &you_key, you_key, (size_t) pthread_getspecific(you_key));
+	printf("them_key before create (%p): %u, me value before create: %zu\n", &them_key, them_key, (size_t) pthread_getspecific(them_key));
+
+	err = pthread_create(&new, NULL, tf, NULL);
+	if (err < 0) {
+		perror("pthread_create");
+		exit(EXIT_FAILURE);
+	}
+
+	printf("me_key after create before join (%p): %u, me value after create before join: %zu\n", &me_key, me_key, (size_t) pthread_getspecific(me_key));
+	printf("you_key after create before join (%p): %u, you value after create before join: %zu\n", &you_key, you_key, (size_t) pthread_getspecific(you_key));
+	printf("them_key after create before join (%p): %u, them value beafter create before join: %zu\n", &them_key, them_key, (size_t) pthread_getspecific(them_key));
+	pthread_join(new, NULL);
+	printf("me_key after join (%p): %u, me value after join: %zu\n", &me_key, me_key, (size_t) pthread_getspecific(me_key));
+	printf("you_key after join (%p): %u, you value after join: %zu\n", &you_key, you_key, (size_t) pthread_getspecific(you_key));
+	printf("them_key after join (%p): %u, them value after join: %zu\n", &them_key, them_key, (size_t) pthread_getspecific(them_key));
+
+	/* This is really not required, but we call it for API completeness. */
+	pthread_key_delete(me_key);
+	pthread_key_delete(you_key);
+	pthread_key_delete(them_key);
+
+	return 0;
+}