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@@ -440,6 +440,94 @@ setup_data: .quad 0 # 64-bit physical pointer to
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pref_address: .quad LOAD_PHYSICAL_ADDR # preferred load addr
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+#
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+# Getting to provably safe in-place decompression is hard. Worst case
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+# behaviours need to be analyzed. Here let's take the decompression of
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+# a gzip-compressed kernel as example, to illustrate it:
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+#
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+# The file layout of gzip compressed kernel is:
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+#
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+# magic[2]
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+# method[1]
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+# flags[1]
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+# timestamp[4]
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+# extraflags[1]
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+# os[1]
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+# compressed data blocks[N]
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+# crc[4] orig_len[4]
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+#
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+# ... resulting in +18 bytes overhead of uncompressed data.
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+#
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+# (For more information, please refer to RFC 1951 and RFC 1952.)
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+#
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+# Files divided into blocks
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+# 1 bit (last block flag)
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+# 2 bits (block type)
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+#
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+# 1 block occurs every 32K -1 bytes or when there 50% compression
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+# has been achieved. The smallest block type encoding is always used.
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+#
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+# stored:
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+# 32 bits length in bytes.
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+#
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+# fixed:
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+# magic fixed tree.
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+# symbols.
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+#
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+# dynamic:
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+# dynamic tree encoding.
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+# symbols.
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+#
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+#
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+# The buffer for decompression in place is the length of the uncompressed
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+# data, plus a small amount extra to keep the algorithm safe. The
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+# compressed data is placed at the end of the buffer. The output pointer
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+# is placed at the start of the buffer and the input pointer is placed
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+# where the compressed data starts. Problems will occur when the output
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+# pointer overruns the input pointer.
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+#
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+# The output pointer can only overrun the input pointer if the input
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+# pointer is moving faster than the output pointer. A condition only
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+# triggered by data whose compressed form is larger than the uncompressed
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+# form.
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+#
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+# The worst case at the block level is a growth of the compressed data
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+# of 5 bytes per 32767 bytes.
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+#
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+# The worst case internal to a compressed block is very hard to figure.
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+# The worst case can at least be bounded by having one bit that represents
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+# 32764 bytes and then all of the rest of the bytes representing the very
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+# very last byte.
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+#
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+# All of which is enough to compute an amount of extra data that is required
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+# to be safe. To avoid problems at the block level allocating 5 extra bytes
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+# per 32767 bytes of data is sufficient. To avoid problems internal to a
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+# block adding an extra 32767 bytes (the worst case uncompressed block size)
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+# is sufficient, to ensure that in the worst case the decompressed data for
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+# block will stop the byte before the compressed data for a block begins.
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+# To avoid problems with the compressed data's meta information an extra 18
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+# bytes are needed. Leading to the formula:
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+#
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+# extra_bytes = (uncompressed_size >> 12) + 32768 + 18 + decompressor_size
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+#
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+# Adding 8 bytes per 32K is a bit excessive but much easier to calculate.
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+# Adding 32768 instead of 32767 just makes for round numbers.
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+# Adding the decompressor_size is necessary as it musht live after all
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+# of the data as well. Last I measured the decompressor is about 14K.
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+# 10K of actual data and 4K of bss.
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+#
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+# Above analysis is for decompressing gzip compressed kernel only. Up to
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+# now 6 different decompressor are supported all together. And among them
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+# xz stores data in chunks and has maximum chunk of 64K. Hence safety
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+# margin should be updated to cover all decompressors so that we don't
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+# need to deal with each of them separately. Please check
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+# the description in lib/decompressor_xxx.c for specific information.
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+#
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+# extra_bytes = (uncompressed_size >> 12) + 65536 + 128
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+#
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+# Note that this calculation, which results in z_extract_offset (below),
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+# is currently generated in compressed/mkpiggy.c
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+
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#define ZO_INIT_SIZE (ZO__end - ZO_startup_32 + ZO_z_extract_offset)
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#define VO_INIT_SIZE (VO__end - VO__text)
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#if ZO_INIT_SIZE > VO_INIT_SIZE
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Baoquan He