[PATCH v2] binfmt_elf: Dump smaller VMAs first in ELF cores

[Brian Mak <makb@juniper.net>]

Large cores may be truncated in some scenarios, such as with daemons
with stop timeouts that are not large enough or lack of disk space. This
impacts debuggability with large core dumps since critical information
necessary to form a usable backtrace, such as stacks and shared library
information, are omitted.

Attempting to find all the VMAs necessary to form a proper backtrace and
then prioritizing those VMAs specifically while core dumping is complex.
So instead, we can mitigate the impact of core dump truncation by
dumping smaller VMAs first, which may be more likely to contain memory
necessary to form a usable backtrace.

By sorting VMAs by dump size and dumping in that order, we have a
simple, yet effective heuristic.

Signed-off-by: Brian Mak <makb@juniper.net>
---

v2: Edited commit message to include more reasoning for sorting VMAs
    Removed conditional VMA sorting with debugfs knob
    Above edits suggested by Eric Biederman <ebiederm@xmission.com>

 fs/binfmt_elf.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 19fa49cd9907..7feadbdd9ee6 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -37,6 +37,7 @@
 #include <linux/elf-randomize.h>
 #include <linux/utsname.h>
 #include <linux/coredump.h>
+#include <linux/sort.h>
 #include <linux/sched.h>
 #include <linux/sched/coredump.h>
 #include <linux/sched/task_stack.h>
@@ -1990,6 +1991,20 @@ static void fill_extnum_info(struct elfhdr *elf, struct elf_shdr *shdr4extnum,
 	shdr4extnum->sh_info = segs;
 }
 
+static int cmp_vma_size(const void *vma_meta_lhs_ptr, const void *vma_meta_rhs_ptr)
+{
+	const struct core_vma_metadata *vma_meta_lhs = *(const struct core_vma_metadata **)
+		vma_meta_lhs_ptr;
+	const struct core_vma_metadata *vma_meta_rhs = *(const struct core_vma_metadata **)
+		vma_meta_rhs_ptr;
+
+	if (vma_meta_lhs->dump_size < vma_meta_rhs->dump_size)
+		return -1;
+	if (vma_meta_lhs->dump_size > vma_meta_rhs->dump_size)
+		return 1;
+	return 0;
+}
+
 /*
  * Actual dumper
  *
@@ -2008,6 +2023,7 @@ static int elf_core_dump(struct coredump_params *cprm)
 	struct elf_shdr *shdr4extnum = NULL;
 	Elf_Half e_phnum;
 	elf_addr_t e_shoff;
+	struct core_vma_metadata **sorted_vmas = NULL;
 
 	/*
 	 * The number of segs are recored into ELF header as 16bit value.
@@ -2071,9 +2087,20 @@ static int elf_core_dump(struct coredump_params *cprm)
 	if (!dump_emit(cprm, phdr4note, sizeof(*phdr4note)))
 		goto end_coredump;
 
+	/* Allocate memory to sort VMAs and sort. */
+	sorted_vmas = kvmalloc_array(cprm->vma_count, sizeof(*sorted_vmas), GFP_KERNEL);
+
+	if (!sorted_vmas)
+		goto end_coredump;
+
+	for (i = 0; i < cprm->vma_count; i++)
+		sorted_vmas[i] = cprm->vma_meta + i;
+
+	sort(sorted_vmas, cprm->vma_count, sizeof(*sorted_vmas), cmp_vma_size, NULL);
+
 	/* Write program headers for segments dump */
 	for (i = 0; i < cprm->vma_count; i++) {
-		struct core_vma_metadata *meta = cprm->vma_meta + i;
+		struct core_vma_metadata *meta = sorted_vmas[i];
 		struct elf_phdr phdr;
 
 		phdr.p_type = PT_LOAD;
@@ -2111,7 +2138,7 @@ static int elf_core_dump(struct coredump_params *cprm)
 	dump_skip_to(cprm, dataoff);
 
 	for (i = 0; i < cprm->vma_count; i++) {
-		struct core_vma_metadata *meta = cprm->vma_meta + i;
+		struct core_vma_metadata *meta = sorted_vmas[i];
 
 		if (!dump_user_range(cprm, meta->start, meta->dump_size))
 			goto end_coredump;
@@ -2129,6 +2156,7 @@ static int elf_core_dump(struct coredump_params *cprm)
 	free_note_info(&info);
 	kfree(shdr4extnum);
 	kfree(phdr4note);
+	kvfree(sorted_vmas);
 	return has_dumped;
 }
 

base-commit: 94ede2a3e9135764736221c080ac7c0ad993dc2d
-- 
2.25.1