[PATCH] vmcore: disable mmap_vmcore() if CONFIG_MMU is not defined

[HATAYAMA Daisuke]

>From Amd's report of a link-time build error in vmcore.c, it turned
out that mmap_vmcore() work overlooked no-MMU configuraiton.

In the current design, it's impossible to implement mmap_vmcore() for
no-MMU configuration since MMU is essential in order to map physically
non-contiguous objects (ELF header, ELF note segment and memory
regions in the 1st kernel pointed to by PT_LOAD entries) into
virtually contiguous user-space in ELF layout.

Hence, this patch disables mmap_vmcore() if CONFIG_MMU is not defined,
returning -ENOSYS.

Another change is to fix the build error by using vmalloc_user()
instead of calling vzalloc() and find_vm_area() in order, by which we
no longer need to call find_vm_area() in vmcore.c that has no
counterpart on non-MMU configuration.

Also, on no-MMU configuration, because we don't export buffer for ELF
note segment to user-space, we use vzalloc() to allocate the buffer.
Therefore, we use differnet functions to allocate the buffer for ELF
note segment. To avoid code duplication, introduce a helper
alloc_elfnotes_buf().

Reported-by: Amd Bergmann <arnd@xxxxxxxx>
Signed-off-by: HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx>
---
 fs/proc/vmcore.c |   57 +++++++++++++++++++++++++++++++++++------------------
 1 files changed, 37 insertions(+), 20 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 0f6db52..2850317 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -195,6 +195,35 @@ static ssize_t read_vmcore(struct file *file, char __user *buffer,
 	return acc;
 }
 
+/**
+ * alloc_elfnotes_buf - allocate buffer for ELF note segment in
+ *                      vmalloc memory
+ *
+ * @notes_sz: size of buffer
+ *
+ * If CONFIG_MMU is defined, use vmalloc_user() to allow users to mmap
+ * the buffer to user-space by means of remap_vmalloc_range().
+ *
+ * If CONFIG_MMU is not defined, use vzalloc() since mmap_vmcore() is
+ * disabled and there's no need to allow users to mmap the buffer.
+ */
+static inline char *alloc_elfnotes_buf(size_t notes_sz)
+{
+#ifdef CONFIG_MMU
+	return vmalloc_user(notes_sz);
+#else
+	return vzalloc(notes_sz);
+#endif
+}
+
+/*
+ * Disable mmap_vmcore() if CONFIG_MMU is not defined. MMU is
+ * essential for mmap_vmcore() in order to map physically
+ * non-contiguous objects (ELF header, ELF note segment and memory
+ * regions in the 1st kernel pointed to by PT_LOAD entries) into
+ * virtually contiguous user-space in ELF layout.
+ */
+#ifdef CONFIG_MMU
 static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
 {
 	size_t size = vma->vm_end - vma->vm_start;
@@ -271,6 +300,12 @@ fail:
 	do_munmap(vma->vm_mm, vma->vm_start, len);
 	return -EAGAIN;
 }
+#else
+static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
+{
+	return -ENOSYS;
+}
+#endif
 
 static const struct file_operations proc_vmcore_operations = {
 	.read		= read_vmcore,
@@ -427,7 +462,6 @@ static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
 	Elf64_Ehdr *ehdr_ptr;
 	Elf64_Phdr phdr;
 	u64 phdr_sz = 0, note_off;
-	struct vm_struct *vm;
 
 	ehdr_ptr = (Elf64_Ehdr *)elfptr;
 
@@ -440,18 +474,10 @@ static int __init merge_note_headers_elf64(char *elfptr, size_t *elfsz,
 		return rc;
 
 	*notes_sz = roundup(phdr_sz, PAGE_SIZE);
-	*notes_buf = vzalloc(*notes_sz);
+	*notes_buf = alloc_elfnotes_buf(*notes_sz);
 	if (!*notes_buf)
 		return -ENOMEM;
 
-	/*
-	 * Allow users to remap ELF note segment buffer on vmalloc memory using
-	 * remap_vmalloc_range.()
-	 */
-	vm = find_vm_area(*notes_buf);
-	BUG_ON(!vm);
-	vm->flags |= VM_USERMAP;
-
 	rc = copy_notes_elf64(ehdr_ptr, *notes_buf);
 	if (rc < 0)
 		return rc;
@@ -615,7 +641,6 @@ static int __init merge_note_headers_elf32(char *elfptr, size_t *elfsz,
 	Elf32_Ehdr *ehdr_ptr;
 	Elf32_Phdr phdr;
 	u64 phdr_sz = 0, note_off;
-	struct vm_struct *vm;
 
 	ehdr_ptr = (Elf32_Ehdr *)elfptr;
 
@@ -628,18 +653,10 @@ static int __init merge_note_headers_elf32(char *elfptr, size_t *elfsz,
 		return rc;
 
 	*notes_sz = roundup(phdr_sz, PAGE_SIZE);
-	*notes_buf = vzalloc(*notes_sz);
+	*notes_buf = alloc_elfnotes_buf(*notes_sz);
 	if (!*notes_buf)
 		return -ENOMEM;
 
-	/*
-	 * Allow users to remap ELF note segment buffer on vmalloc memory using
-	 * remap_vmalloc_range()
-	 */
-	vm = find_vm_area(*notes_buf);
-	BUG_ON(!vm);
-	vm->flags |= VM_USERMAP;
-
 	rc = copy_notes_elf32(ehdr_ptr, *notes_buf);
 	if (rc < 0)
 		return rc;
-- 
1.7.7.6

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