[PATCH 14/19] mm: Add user_landing in mm_struct

[Dmitry Safonov]

Instead of having every architecture to define vdso_base/vdso_addr etc,
provide a generic mechanism to track landing in userspace.
It'll minimize per-architecture difference, the number of callbacks to
provide.

Originally, it started from thread [1] where the need for .close()
callback on vm_special_mapping was pointed, this generic code besides
removing duplicated .mremap() callbacks provides a cheaper way to
support munmap() on vdso mappings without introducing .close() callbacks
for every architecture (with would bring even more code duplication).

[1]: https://lore.kernel.org/linux-arch/CAJwJo6ZANqYkSHbQ+3b+Fi_VT80MtrzEV5yreQAWx-L8j8x2zA@xxxxxxxxxxxxxx/
Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/resctrl/pseudo_lock.c |  3 ++-
 fs/aio.c                                  |  3 ++-
 include/linux/mm.h                        |  3 ++-
 include/linux/mm_types.h                  | 10 ++++++++++
 mm/Kconfig                                |  3 +++
 mm/mmap.c                                 | 19 ++++++++++++++++++-
 mm/mremap.c                               |  2 +-
 7 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
index e916646adc69..786c97203bf6 100644
--- a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
+++ b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
@@ -1458,7 +1458,8 @@ static int pseudo_lock_dev_release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-static int pseudo_lock_dev_mremap(struct vm_area_struct *area, unsigned long flags)
+static int pseudo_lock_dev_mremap(struct vm_area_struct *old_vma,
+		struct vm_area_struct *new_vma, unsigned long flags)
 {
 	/* Not supported */
 	return -EINVAL;
diff --git a/fs/aio.c b/fs/aio.c
index d1dad4cd860f..2695dc9ed46f 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -324,7 +324,8 @@ static void aio_free_ring(struct kioctx *ctx)
 	}
 }
 
-static int aio_ring_mremap(struct vm_area_struct *vma, unsigned long flags)
+static int aio_ring_mremap(struct vm_area_struct *old_vma,
+			   struct vm_area_struct *vma, unsigned long flags)
 {
 	struct file *file = vma->vm_file;
 	struct mm_struct *mm = vma->vm_mm;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 427911d2c83e..4b0f97a289b3 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -559,7 +559,8 @@ struct vm_operations_struct {
 	void (*close)(struct vm_area_struct * area);
 	/* Called any time before splitting to check if it's allowed */
 	int (*may_split)(struct vm_area_struct *area, unsigned long addr);
-	int (*mremap)(struct vm_area_struct *area, unsigned long flags);
+	int (*mremap)(struct vm_area_struct *old_vma,
+			struct vm_area_struct *new_vma, unsigned long flags);
 	vm_fault_t (*fault)(struct vm_fault *vmf);
 	vm_fault_t (*huge_fault)(struct vm_fault *vmf,
 			enum page_entry_size pe_size);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index b035caff6abe..f888257e973a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -488,6 +488,16 @@ struct mm_struct {
 
 		/* Architecture-specific MM context */
 		mm_context_t context;
+#ifdef CONFIG_ARCH_HAS_USER_LANDING
+		/*
+		 * Address of special mapping VMA to land after processing
+		 * a signal. Reads are unprotected: if a thread unmaps or
+		 * mremaps the mapping while another thread is processing
+		 * a signal, it can segfault while landing.
+		 */
+		void __user *user_landing;
+#endif
+#define UNMAPPED_USER_LANDING TASK_SIZE_MAX
 
 		unsigned long flags; /* Must use atomic bitops to access */
 
diff --git a/mm/Kconfig b/mm/Kconfig
index 01b0ae0cd9d3..d43b61a21be8 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -883,4 +883,7 @@ config ARCH_HAS_HUGEPD
 config MAPPING_DIRTY_HELPERS
         bool
 
+config ARCH_HAS_USER_LANDING
+	bool
+
 endmenu
diff --git a/mm/mmap.c b/mm/mmap.c
index 2376f3972f13..8a17ffdedacb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3410,11 +3410,25 @@ void vm_stat_account(struct mm_struct *mm, vm_flags_t flags, long npages)
 
 static vm_fault_t special_mapping_fault(struct vm_fault *vmf);
 
+static void update_user_landing(struct vm_area_struct *old_vma,
+				unsigned long new_addr)
+{
+#ifdef CONFIG_ARCH_HAS_USER_LANDING
+	struct mm_struct *mm = old_vma->vm_mm;
+
+	if (WARN_ON_ONCE(!mm))
+		return;
+	if (old_vma->vm_start == (unsigned long)mm->user_landing)
+		mm->user_landing = (void __user *)new_addr;
+#endif
+}
+
 /*
  * Having a close hook prevents vma merging regardless of flags.
  */
 static void special_mapping_close(struct vm_area_struct *vma)
 {
+	update_user_landing(vma, UNMAPPED_USER_LANDING);
 }
 
 static const char *special_mapping_name(struct vm_area_struct *vma)
@@ -3422,7 +3436,8 @@ static const char *special_mapping_name(struct vm_area_struct *vma)
 	return ((struct vm_special_mapping *)vma->vm_private_data)->name;
 }
 
-static int special_mapping_mremap(struct vm_area_struct *new_vma,
+static int special_mapping_mremap(struct vm_area_struct *old_vma,
+				  struct vm_area_struct *new_vma,
 				  unsigned long flags)
 {
 	struct vm_special_mapping *sm = new_vma->vm_private_data;
@@ -3436,6 +3451,8 @@ static int special_mapping_mremap(struct vm_area_struct *new_vma,
 	if (sm->mremap)
 		sm->mremap(sm, new_vma);
 
+	update_user_landing(old_vma, new_vma->vm_start);
+
 	return 0;
 }
 
diff --git a/mm/mremap.c b/mm/mremap.c
index c5590afe7165..9595f6b72101 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -543,7 +543,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 	if (moved_len < old_len) {
 		err = -ENOMEM;
 	} else if (vma->vm_ops && vma->vm_ops->mremap) {
-		err = vma->vm_ops->mremap(new_vma, flags);
+		err = vma->vm_ops->mremap(vma, new_vma, flags);
 	}
 
 	if (unlikely(err)) {
-- 
2.28.0