[PATCH -mm 3/3] ELF, coredump: allow process with empty address space to coredump

[Alexey Dobriyan]

Unmapping whole address space at once with

	munmap(0, (1ULL<<47) - 4096)

or equivalent will create empty coredump.

It is silly way to exit, however registers content may still be useful.

The right to coredump is fundamental right of a process!

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
---

 fs/binfmt_elf.c                                  |   10 +++++-
 tools/testing/selftests/exec/Makefile            |    1 
 tools/testing/selftests/exec/coredump-zero-vma.c |   38 +++++++++++++++++++++++
 3 files changed, 48 insertions(+), 1 deletion(-)

--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1595,6 +1595,10 @@ static int fill_files_note(struct memelfnote *note)
 	if (size >= MAX_FILE_NOTE_SIZE) /* paranoia check */
 		return -EINVAL;
 	size = round_up(size, PAGE_SIZE);
+	/*
+	 * "size" can be 0 here legitimately.
+	 * Let it ENOMEM and omit NT_FILE section which will be empty anyway.
+	 */
 	data = kvmalloc(size, GFP_KERNEL);
 	if (ZERO_OR_NULL_PTR(data))
 		return -ENOMEM;
@@ -2257,9 +2261,13 @@ static int elf_core_dump(struct coredump_params *cprm)
 
 	dataoff = offset = roundup(offset, ELF_EXEC_PAGESIZE);
 
+	/*
+	 * Zero vma process will get ZERO_SIZE_PTR here.
+	 * Let coredump continue for register state at least.
+	 */
 	vma_filesz = kvmalloc(array_size(sizeof(*vma_filesz), (segs - 1)),
 			      GFP_KERNEL);
-	if (ZERO_OR_NULL_PTR(vma_filesz))
+	if (!vma_filesz)
 		goto end_coredump;
 
 	for (i = 0, vma = first_vma(current, gate_vma); vma != NULL;
--- a/tools/testing/selftests/exec/Makefile
+++ b/tools/testing/selftests/exec/Makefile
@@ -8,6 +8,7 @@ TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir
 # Makefile is a run-time dependency, since it's accessed by the execveat test
 TEST_FILES := Makefile
 
+TEST_GEN_PROGS += coredump-zero-vma
 TEST_GEN_PROGS += recursion-depth
 
 EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*
new file mode 100644
--- /dev/null
+++ b/tools/testing/selftests/exec/coredump-zero-vma.c
@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2019 Alexey Dobriyan
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+/* Test that unmapping whole address space at once doesn't do anything funny. */
+#include <stddef.h>
+#include <sys/mman.h>
+#include <sys/resource.h>
+
+#ifdef __x86_64__
+static const size_t len = ((size_t)1 << 47) - 4096;
+#endif
+
+#ifdef __x86_64__
+int main(void)
+{
+	setrlimit(RLIMIT_CORE, &(struct rlimit){RLIM_INFINITY, RLIM_INFINITY});
+	munmap(NULL, len);
+	/* And... it's gone! */
+	return 1;
+}
+#else
+int main(void)
+{
+	return 4;
+}
+#endif