[PATCH v2 1/2] asm/sections: Add helpers to check for section data

[PATCH v2 1/2] asm/sections: Add helpers to check for section data

[Thierry Reding]

From: Thierry Reding <treding@nvidia.com>

Add a helper to check if an object (given an address and a size) is part
of a section (given beginning and end addresses). For convenience, also
provide a helper that performs this check for __init data using the
__init_begin and __init_end limits.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v2:
- use less ambiguous function names
- add comments to clarify further
---
 include/asm-generic/sections.h | 65 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index b58fd667f87b..af0254c09424 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -4,6 +4,7 @@
 /* References to section boundaries */
 
 #include <linux/compiler.h>
+#include <linux/types.h>
 
 /*
  * Usage guidelines:
@@ -63,4 +64,68 @@ static inline int arch_is_kernel_data(unsigned long addr)
 }
 #endif
 
+/**
+ * memory_contains - checks if an object is contained within a memory region
+ * @begin: virtual address of the beginning of the memory region
+ * @end: virtual address of the end of the memory region
+ * @virt: virtual address of the memory object
+ * @size: size of the memory object
+ *
+ * Returns: true if the object specified by @virt and @size is entirely
+ * contained within the memory region defined by @begin and @end, false
+ * otherwise.
+ */
+static inline bool memory_contains(void *begin, void *end, void *virt,
+				   size_t size)
+{
+	return virt >= begin && virt + size <= end;
+}
+
+/**
+ * memory_intersects - checks if the region occupied by an object intersects
+ *                     with another memory region
+ * @begin: virtual address of the beginning of the memory regien
+ * @end: virtual address of the end of the memory region
+ * @virt: virtual address of the memory object
+ * @size: size of the memory object
+ *
+ * Returns: true if an object's memory region, specified by @virt and @size,
+ * intersects with the region specified by @begin and @end, false otherwise.
+ */
+static inline bool memory_intersects(void *begin, void *end, void *virt,
+				     size_t size)
+{
+	void *vend = virt + size;
+
+	return (virt >= begin && virt < end) || (vend >= begin && vend < end);
+}
+
+/**
+ * init_section_contains - checks if an object is contained within the init
+ *                         section
+ * @virt: virtual address of the memory object
+ * @size: size of the memory object
+ *
+ * Returns: true if the object specified by @virt and @size is entirely
+ * contained within the init section, false otherwise.
+ */
+static inline bool init_section_contains(void *virt, size_t size)
+{
+	return memory_contains(__init_begin, __init_end, virt, size);
+}
+
+/**
+ * init_section_intersects - checks if the region occupied by an object
+ *                           intersects with the init section
+ * @virt: virtual address of the memory object
+ * @size: size of the memory object
+ *
+ * Returns: true if an object's memory region, specified by @virt and @size,
+ * intersects with the init section, false otherwise.
+ */
+static inline bool init_section_intersects(void *virt, size_t size)
+{
+	return memory_intersects(__init_begin, __init_end, virt, size);
+}
+
 #endif /* _ASM_GENERIC_SECTIONS_H_ */
-- 
2.5.0

[PATCH v2 2/2] printk: Only unregister boot consoles when necessary

[Thierry Reding]

From: Thierry Reding <treding@nvidia.com>

Boot consoles are typically replaced by proper consoles during the boot
process. This can be problematic if the boot console data is part of the
init section that is reclaimed late during boot. If the proper console
does not register before this point in time, the boot console will need
to be removed (so that the freed memory is not accessed), leaving the
system without output for some time.

There are various reasons why the proper console may not register early
enough, such as deferred probe or the driver being a loadable module. If
that happens, there is some amount of time where no console messages are
visible to the user, which in turn can mean that they won't see crashes
or other potentially useful information.

To avoid this situation, only remove the boot console when it resides in
the init section. Code exists to replace the boot console by the proper
console when it is registered, keeping a seamless transition between the
boot and proper consoles.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v2:
- add comments with the rationale for this change
---
 kernel/printk/printk.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2ce8826f1053..0c9f02506169 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -48,6 +48,7 @@
 #include <linux/uio.h>
 
 #include <asm/uaccess.h>
+#include <asm-generic/sections.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/printk.h>
@@ -2658,13 +2659,36 @@ int unregister_console(struct console *console)
 }
 EXPORT_SYMBOL(unregister_console);
 
+/*
+ * Some boot consoles access data that is in the init section and which will
+ * be discarded after the initcalls have been run. To make sure that no code
+ * will access this data, unregister the boot consoles in a late initcall.
+ *
+ * If for some reason, such as deferred probe or the driver being a loadable
+ * module, the real console hasn't registered yet at this point, there will
+ * be a brief interval in which no messages are logged to the console, which
+ * makes it difficult to diagnose problems that occur during this time.
+ *
+ * To mitigate this problem somewhat, only unregister consoles whose memory
+ * intersects with the init section. Note that code exists elsewhere to get
+ * rid of the boot console as soon as the proper console shows up, so there
+ * won't be side-effects from postponing the removal.
+ */
 static int __init printk_late_init(void)
 {
 	struct console *con;
 
 	for_each_console(con) {
 		if (!keep_bootcon && con->flags & CON_BOOT) {
-			unregister_console(con);
+			/*
+			 * Make sure to unregister boot consoles whose data
+			 * resides in the init section before the init section
+			 * is discarded. Boot consoles whose data will stick
+			 * around will automatically be unregistered when the
+			 * proper console replaces them.
+			 */
+			if (init_section_intersects(con, sizeof(*con)))
+				unregister_console(con);
 		}
 	}
 	hotcpu_notifier(console_cpu_notify, 0);
-- 
2.5.0