journal: adapt for new improved LZ4_decompress_safe_partial()

With lz4 1.8.3, this function can now decompress partial results into a smaller
buffer. The release news don't say anything interesting, but the test case that
was previously failing now works OK.

Fixes systemd#10259.

A test is added. It shows that with *older* lz4, a partial decompression can
occur with the returned size smaller then the requested number of bytes _and_
smaller then the size of the compressed data:

(lz4-libs-1.8.2-1.fc29.x86_64)
Compressed 4194304 → 16464
Decompressed → 4194304
Decompressed partial 12/4194304 → 4194304
Decompressed partial 1/1 → -2 (bad)
Decompressed partial 2/2 → -2 (bad)
Decompressed partial 3/3 → -2 (bad)
Decompressed partial 4/4 → -2 (bad)
Decompressed partial 5/5 → -2 (bad)
Decompressed partial 6/6 → 6 (good)
Decompressed partial 7/7 → 6 (good)
Decompressed partial 8/8 → 6 (good)
Decompressed partial 9/9 → 6 (good)
Decompressed partial 10/10 → 6 (good)
Decompressed partial 11/11 → 6 (good)
Decompressed partial 12/12 → 6 (good)
Decompressed partial 13/13 → 6 (good)
Decompressed partial 14/14 → 6 (good)
Decompressed partial 15/15 → 6 (good)
Decompressed partial 16/16 → 6 (good)
Decompressed partial 17/17 → 6 (good)
Decompressed partial 18/18 → -16459 (bad)

(lz4-libs-1.8.3-1.fc29.x86_64)
Compressed 4194304 → 16464
Decompressed → 4194304
Decompressed partial 12/4194304 → 12
Decompressed partial 1/1 → 1 (good)
Decompressed partial 2/2 → 2 (good)
Decompressed partial 3/3 → 3 (good)
Decompressed partial 4/4 → 4 (good)
...

If we got such a short "successful" decompression in decompress_startswith() as
implemented before this patch, we could be confused and return a false negative
result. But it turns out that this only occurs with small output buffer
sizes. We use greedy_realloc() to manager the buffer, so it is always at least
64 bytes. I couldn't hit a case where decompress_startswith() would actually
return a bogus result. But since the lack of proof is not conclusive, the code
for *older* lz4 is changed too, just to be safe. We cannot rule out that on a
different architecture or with some unlucky compressed string we could hit this
corner case.

The fallback code is guarded by a version check. The check uses a function not
the compile-time define, because there was no soversion bump in lz4 or new
symbols, and we could be compiled against a newer lz4 and linked at runtime
with an older one. (This happens routinely e.g. when somebody upgrades a subset
of distro packages.)

Author: keszybz

src/journal/compress.c

@@ -290,7 +290,6 @@ int decompress_startswith_lz4(const void *src, uint64_t src_size,
          * prefix */
 
         int r;
-        size_t size;
 
         assert(src);
         assert(src_size > 0);
@@ -307,23 +306,37 @@ int decompress_startswith_lz4(const void *src, uint64_t src_size,
 
         r = LZ4_decompress_safe_partial((char*)src + 8, *buffer, src_size - 8,
                                         prefix_len + 1, *buffer_size);
-        if (r >= 0)
-                size = (unsigned) r;
-        else {
-                /* lz4 always tries to decode full "sequence", so in
-                 * pathological cases might need to decompress the
-                 * full field. */
+        /* One lz4 < 1.8.3, we might get "failure" (r < 0), or "success" where
+         * just a part of the buffer is decompressed. But if we get a smaller
+         * amount of bytes than requested, we don't know whether there isn't enough
+         * data to fill the requested size or whether we just got a partial answer.
+         */
+        if (r < 0 || (size_t) r < prefix_len + 1) {
+                size_t size;
+
+                if (LZ4_versionNumber() >= 10803)
+                        /* We trust that the newer lz4 decompresses the number of bytes we
+                         * requested if available in the compressed string. */
+                        return 0;
+
+                if (r > 0)
+                        /* Compare what we have first, in case of mismatch we can
+                         * shortcut the full comparison. */
+                        if (memcmp(*buffer, prefix, r) != 0)
+                                return 0;
+
+                /* Before version 1.8.3, lz4 always tries to decode full a "sequence",
+                 * so in pathological cases might need to decompress the full field. */
                 r = decompress_blob_lz4(src, src_size, buffer, buffer_size, &size, 0);
                 if (r < 0)
                         return r;
-        }
 
-        if (size >= prefix_len + 1)
-                return memcmp(*buffer, prefix, prefix_len) == 0 &&
-                        ((const uint8_t*) *buffer)[prefix_len] == extra;
-        else
-                return 0;
+                if (size < prefix_len + 1)
+                        return 0;
+        }
 
+        return memcmp(*buffer, prefix, prefix_len) == 0 &&
+                ((const uint8_t*) *buffer)[prefix_len] == extra;
 #else
         return -EPROTONOSUPPORT;
 #endif

src/journal/test-compress.c

@@ -224,13 +224,13 @@ static void test_compress_stream(int compression,
 
 #if HAVE_LZ4
 static void test_lz4_decompress_partial(void) {
-        char buf[20000];
+        char buf[20000], buf2[100];
         size_t buf_size = sizeof(buf), compressed;
         int r;
         _cleanup_free_ char *huge = NULL;
 
 #define HUGE_SIZE (4096*1024)
-        huge = malloc(HUGE_SIZE);
+        assert_se(huge = malloc(HUGE_SIZE));
         memset(huge, 'x', HUGE_SIZE);
         memcpy(huge, "HUGE=", 5);
 
@@ -249,14 +249,15 @@ static void test_lz4_decompress_partial(void) {
         assert_se(r >= 0);
         log_info("Decompressed partial %i/%i → %i", 12, HUGE_SIZE, r);
 
-        /* We expect this to fail, because that's how current lz4 works. If this
-         * call succeeds, then lz4 has been fixed, and we need to change our code.
-         */
-        r = LZ4_decompress_safe_partial(buf, huge,
-                                        compressed,
-                                        12, HUGE_SIZE-1);
-        assert_se(r < 0);
-        log_info("Decompressed partial %i/%i → %i", 12, HUGE_SIZE-1, r);
+        for (size_t size = 1; size < sizeof(buf2); size++) {
+                /* This failed in older lz4s but works in newer ones. */
+                r = LZ4_decompress_safe_partial(buf, buf2, compressed, size, size);
+                log_info("Decompressed partial %zu/%zu → %i (%s)", size, size, r,
+                                                                   r < 0 ? "bad" : "good");
+                if (r >= 0 && LZ4_versionNumber() >= 10803)
+                        /* lz4 <= 1.8.2 should fail that test, let's only check for newer ones */
+                        assert_se(memcmp(buf2, huge, r) == 0);
+        }
 }
 #endif