181 lines
7.5 KiB
Diff
181 lines
7.5 KiB
Diff
From 696d56fc75e72f47e4d3232a2140fac10b6b44de Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
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Date: Mon, 29 Oct 2018 14:55:33 +0100
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Subject: [PATCH] journal: adapt for new improved LZ4_decompress_safe_partial()
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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With lz4 1.8.3, this function can now decompress partial results into a smaller
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buffer. The release news don't say anything interesting, but the test case that
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was previously failing now works OK.
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Fixes #10259.
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A test is added. It shows that with *older* lz4, a partial decompression can
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occur with the returned size smaller then the requested number of bytes _and_
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smaller then the size of the compressed data:
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(lz4-libs-1.8.2-1.fc29.x86_64)
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Compressed 4194304 → 16464
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Decompressed → 4194304
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Decompressed partial 12/4194304 → 4194304
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Decompressed partial 1/1 → -2 (bad)
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Decompressed partial 2/2 → -2 (bad)
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Decompressed partial 3/3 → -2 (bad)
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Decompressed partial 4/4 → -2 (bad)
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Decompressed partial 5/5 → -2 (bad)
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Decompressed partial 6/6 → 6 (good)
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Decompressed partial 7/7 → 6 (good)
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Decompressed partial 8/8 → 6 (good)
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Decompressed partial 9/9 → 6 (good)
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Decompressed partial 10/10 → 6 (good)
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Decompressed partial 11/11 → 6 (good)
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Decompressed partial 12/12 → 6 (good)
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Decompressed partial 13/13 → 6 (good)
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Decompressed partial 14/14 → 6 (good)
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Decompressed partial 15/15 → 6 (good)
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Decompressed partial 16/16 → 6 (good)
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Decompressed partial 17/17 → 6 (good)
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Decompressed partial 18/18 → -16459 (bad)
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(lz4-libs-1.8.3-1.fc29.x86_64)
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Compressed 4194304 → 16464
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Decompressed → 4194304
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Decompressed partial 12/4194304 → 12
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Decompressed partial 1/1 → 1 (good)
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Decompressed partial 2/2 → 2 (good)
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Decompressed partial 3/3 → 3 (good)
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Decompressed partial 4/4 → 4 (good)
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...
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If we got such a short "successful" decompression in decompress_startswith() as
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implemented before this patch, we could be confused and return a false negative
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result. But it turns out that this only occurs with small output buffer
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sizes. We use greedy_realloc() to manager the buffer, so it is always at least
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64 bytes. I couldn't hit a case where decompress_startswith() would actually
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return a bogus result. But since the lack of proof is not conclusive, the code
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for *older* lz4 is changed too, just to be safe. We cannot rule out that on a
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different architecture or with some unlucky compressed string we could hit this
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corner case.
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The fallback code is guarded by a version check. The check uses a function not
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the compile-time define, because there was no soversion bump in lz4 or new
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symbols, and we could be compiled against a newer lz4 and linked at runtime
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with an older one. (This happens routinely e.g. when somebody upgrades a subset
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of distro packages.)
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(cherry picked from commit e41ef6fd0027d3619dc1cf062100b2d224d0ee7e)
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Resolves: #1843871
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---
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src/journal/compress.c | 39 ++++++++++++++++++++++++-------------
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src/journal/test-compress.c | 21 ++++++++++----------
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2 files changed, 37 insertions(+), 23 deletions(-)
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diff --git a/src/journal/compress.c b/src/journal/compress.c
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index a4a5e63840..e95ce2bcaa 100644
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--- a/src/journal/compress.c
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+++ b/src/journal/compress.c
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@@ -290,7 +290,6 @@ int decompress_startswith_lz4(const void *src, uint64_t src_size,
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* prefix */
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int r;
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- size_t size;
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assert(src);
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assert(src_size > 0);
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@@ -307,23 +306,37 @@ int decompress_startswith_lz4(const void *src, uint64_t src_size,
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r = LZ4_decompress_safe_partial((char*)src + 8, *buffer, src_size - 8,
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prefix_len + 1, *buffer_size);
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- if (r >= 0)
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- size = (unsigned) r;
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- else {
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- /* lz4 always tries to decode full "sequence", so in
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- * pathological cases might need to decompress the
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- * full field. */
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+ /* One lz4 < 1.8.3, we might get "failure" (r < 0), or "success" where
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+ * just a part of the buffer is decompressed. But if we get a smaller
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+ * amount of bytes than requested, we don't know whether there isn't enough
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+ * data to fill the requested size or whether we just got a partial answer.
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+ */
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+ if (r < 0 || (size_t) r < prefix_len + 1) {
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+ size_t size;
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+
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+ if (LZ4_versionNumber() >= 10803)
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+ /* We trust that the newer lz4 decompresses the number of bytes we
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+ * requested if available in the compressed string. */
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+ return 0;
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+
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+ if (r > 0)
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+ /* Compare what we have first, in case of mismatch we can
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+ * shortcut the full comparison. */
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+ if (memcmp(*buffer, prefix, r) != 0)
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+ return 0;
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+
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+ /* Before version 1.8.3, lz4 always tries to decode full a "sequence",
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+ * so in pathological cases might need to decompress the full field. */
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r = decompress_blob_lz4(src, src_size, buffer, buffer_size, &size, 0);
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if (r < 0)
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return r;
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- }
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- if (size >= prefix_len + 1)
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- return memcmp(*buffer, prefix, prefix_len) == 0 &&
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- ((const uint8_t*) *buffer)[prefix_len] == extra;
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- else
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- return 0;
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+ if (size < prefix_len + 1)
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+ return 0;
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+ }
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+ return memcmp(*buffer, prefix, prefix_len) == 0 &&
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+ ((const uint8_t*) *buffer)[prefix_len] == extra;
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#else
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return -EPROTONOSUPPORT;
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#endif
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diff --git a/src/journal/test-compress.c b/src/journal/test-compress.c
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index 65cd3fbfeb..f60c4ae3d7 100644
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--- a/src/journal/test-compress.c
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+++ b/src/journal/test-compress.c
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@@ -223,13 +223,13 @@ static void test_compress_stream(int compression,
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#if HAVE_LZ4
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static void test_lz4_decompress_partial(void) {
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- char buf[20000];
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+ char buf[20000], buf2[100];
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size_t buf_size = sizeof(buf), compressed;
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int r;
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_cleanup_free_ char *huge = NULL;
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#define HUGE_SIZE (4096*1024)
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- huge = malloc(HUGE_SIZE);
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+ assert_se(huge = malloc(HUGE_SIZE));
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memset(huge, 'x', HUGE_SIZE);
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memcpy(huge, "HUGE=", 5);
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@@ -248,14 +248,15 @@ static void test_lz4_decompress_partial(void) {
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assert_se(r >= 0);
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log_info("Decompressed partial %i/%i → %i", 12, HUGE_SIZE, r);
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- /* We expect this to fail, because that's how current lz4 works. If this
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- * call succeeds, then lz4 has been fixed, and we need to change our code.
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- */
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- r = LZ4_decompress_safe_partial(buf, huge,
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- compressed,
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- 12, HUGE_SIZE-1);
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- assert_se(r < 0);
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- log_info("Decompressed partial %i/%i → %i", 12, HUGE_SIZE-1, r);
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+ for (size_t size = 1; size < sizeof(buf2); size++) {
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+ /* This failed in older lz4s but works in newer ones. */
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+ r = LZ4_decompress_safe_partial(buf, buf2, compressed, size, size);
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+ log_info("Decompressed partial %zu/%zu → %i (%s)", size, size, r,
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+ r < 0 ? "bad" : "good");
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+ if (r >= 0 && LZ4_versionNumber() >= 10803)
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+ /* lz4 <= 1.8.2 should fail that test, let's only check for newer ones */
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+ assert_se(memcmp(buf2, huge, r) == 0);
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+ }
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}
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#endif
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