[PATCH] kexec_file: skip checksum verification when relocations aren't needed

[Michal Clapinski]

Checksum verification is needed
1. for crash kernels. In a crash, we can't be sure the kernel is
   intact.
2. if we're worried about relocating the kernel into a region used by
   some DMA that wasn't properly cancelled.

If we used CMA to allocate segments then
1. we're not working with a crash kernel.
2. relocations are not going to happen.

Therefore, we can safely disable checksum verification.

Instead of adding a new variable to purgatory, just skip adding regions
and save the default value of SHA256 hash.

Saves ~250ms on my 4.0 GHz CPU.

Signed-off-by: Michal Clapinski <mclapinski at google.com>
---
 kernel/kexec_file.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 2bfbb2d144e6..2dc8b0435fe6 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -808,6 +808,7 @@ static int kexec_calculate_store_digests(struct kimage *image)
 	void *zero_buf;
 	struct kexec_sha_region *sha_regions;
 	struct purgatory_info *pi = &image->purgatory_info;
+	bool can_skip_checksum = true;
 
 	if (!IS_ENABLED(CONFIG_ARCH_SUPPORTS_KEXEC_PURGATORY))
 		return 0;
@@ -822,6 +823,23 @@ static int kexec_calculate_store_digests(struct kimage *image)
 
 	sha256_init(&sctx);
 
+	/*
+	 * If all segments were loaded into contiguous memory, there will be no
+	 * relocations. In that case there is no risk of memory corruption by
+	 * uncancelled DMA and we can skip checksum calculation.
+	 */
+	for (i = 0; i < image->nr_segments; i++) {
+		if (!image->segment_cma[i]) {
+			can_skip_checksum = false;
+			break;
+		}
+	}
+
+	if (can_skip_checksum) {
+		pr_info("disabling checksum verification in purgatory\n");
+		goto skip_checksum;
+	}
+
 	for (j = i = 0; i < image->nr_segments; i++) {
 		struct kexec_segment *ksegment;
 
@@ -867,6 +885,7 @@ static int kexec_calculate_store_digests(struct kimage *image)
 		j++;
 	}
 
+skip_checksum:
 	sha256_final(&sctx, digest);
 
 	ret = kexec_purgatory_get_set_symbol(image, "purgatory_sha_regions",
-- 
2.54.0.929.g9b7fa37559-goog