[PATCH v4 05/20] x86, kaskr: Update the description for decompressor worst case
From: Baoquan He
Date: Tue Mar 22 2016 - 03:37:11 EST
Current analysis is only for gzip decompressor only. In fact we can
support 6 different decompressor. Update the description to cover
all of them. Meanwhile fix several typos.
Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
arch/x86/boot/compressed/misc.c | 26 +++++++++++++++++---------
1 file changed, 17 insertions(+), 9 deletions(-)
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index e2a998f..e7c6fdf 100644
@@ -19,11 +19,13 @@
- * Getting to provable safe in place decompression is hard.
- * Worst case behaviours need to be analyzed.
- * Background information:
+ * Getting to provable safe in place decompression is hard. Worst case
+ * behaviours need be analyzed. Here let's take decompressing gzip-compressed
+ * kernel as example to illustrate it.
+ * The file layout of gzip compressed kernel is as follows. For more information,
+ * please refer to rfc 1951 and rfc 1952.
- * The file layout is:
@@ -70,13 +72,13 @@
* of 5 bytes per 32767 bytes.
* The worst case internal to a compressed block is very hard to figure.
- * The worst case can at least be boundined by having one bit that represents
+ * The worst case can at least be bounded by having one bit that represents
* 32764 bytes and then all of the rest of the bytes representing the very
* very last byte.
* All of which is enough to compute an amount of extra data that is required
* to be safe. To avoid problems at the block level allocating 5 extra bytes
- * per 32767 bytes of data is sufficient. To avoind problems internal to a
+ * per 32767 bytes of data is sufficient. To avoid problems internal to a
* block adding an extra 32767 bytes (the worst case uncompressed block size)
* is sufficient, to ensure that in the worst case the decompressed data for
* block will stop the byte before the compressed data for a block begins.
@@ -88,11 +90,17 @@
* Adding 8 bytes per 32K is a bit excessive but much easier to calculate.
* Adding 32768 instead of 32767 just makes for round numbers.
+ * Above analysis is for decompressing gzip compressed kernel only. Up to
+ * now 6 different decompressor are supported all together. And among them
+ * xz stores data in chunks and has maximum chunk of 64K. Hence safety
+ * margin should be updated to cover all decompressors so that we don't
+ * need to deal with each of them separately. Please check
+ * the description in lib/decompressor_xxx.c for specific information.
+ * extra_bytes = (uncompressed_size >> 12) + 65536 + 128.
- * gzip declarations
#define STATIC static