From 4ac2fd9b4aabe72f8bc652b71d2fcd9d952e8093 Mon Sep 17 00:00:00 2001 From: Patrick Steinhardt Date: Fri, 23 Aug 2024 16:12:46 +0200 Subject: reftable/stack: fix broken refnames in `write_n_ref_tables()` The `write_n_ref_tables()` helper function writes N references in separate tables. We never reset the computed name of those references though, leading us to end up with unexpected names. Fix this by resetting the buffer. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano --- reftable/stack_test.c | 1 + 1 file changed, 1 insertion(+) (limited to 'reftable/stack_test.c') diff --git a/reftable/stack_test.c b/reftable/stack_test.c index 42044ed8a3..de0669b7b8 100644 --- a/reftable/stack_test.c +++ b/reftable/stack_test.c @@ -125,6 +125,7 @@ static void write_n_ref_tables(struct reftable_stack *st, .value_type = REFTABLE_REF_VAL1, }; + strbuf_reset(&buf); strbuf_addf(&buf, "refs/heads/branch-%04u", (unsigned) i); ref.refname = buf.buf; set_test_hash(ref.value.val1, i); -- cgit 1.2.3-korg From d857469d850b5e020de181ec07806872531d35e7 Mon Sep 17 00:00:00 2001 From: Patrick Steinhardt Date: Fri, 23 Aug 2024 16:12:48 +0200 Subject: reftable/reader: introduce refcounting It was recently reported that concurrent reads and writes may cause the reftable backend to segfault. The root cause of this is that we do not properly keep track of reftable readers across reloads. Suppose that you have a reftable iterator and then decide to reload the stack while iterating through the iterator. When the stack has been rewritten since we have created the iterator, then we would end up discarding a subset of readers that may still be in use by the iterator. The consequence is that we now try to reference deallocated memory, which of course segfaults. One way to trigger this is in t5616, where some background maintenance jobs have been leaking from one test into another. This leads to stack traces like the following one: + git -c protocol.version=0 -C pc1 fetch --filter=blob:limit=29999 --refetch origin AddressSanitizer:DEADLYSIGNAL ================================================================= ==657994==ERROR: AddressSanitizer: SEGV on unknown address 0x7fa0f0ec6089 (pc 0x55f23e52ddf9 bp 0x7ffe7bfa1700 sp 0x7ffe7bfa1700 T0) ==657994==The signal is caused by a READ memory access. #0 0x55f23e52ddf9 in get_var_int reftable/record.c:29 #1 0x55f23e53295e in reftable_decode_keylen reftable/record.c:170 #2 0x55f23e532cc0 in reftable_decode_key reftable/record.c:194 #3 0x55f23e54e72e in block_iter_next reftable/block.c:398 #4 0x55f23e5573dc in table_iter_next_in_block reftable/reader.c:240 #5 0x55f23e5573dc in table_iter_next reftable/reader.c:355 #6 0x55f23e5573dc in table_iter_next reftable/reader.c:339 #7 0x55f23e551283 in merged_iter_advance_subiter reftable/merged.c:69 #8 0x55f23e55169e in merged_iter_next_entry reftable/merged.c:123 #9 0x55f23e55169e in merged_iter_next_void reftable/merged.c:172 #10 0x55f23e537625 in reftable_iterator_next_ref reftable/generic.c:175 #11 0x55f23e2cf9c6 in reftable_ref_iterator_advance refs/reftable-backend.c:464 #12 0x55f23e2d996e in ref_iterator_advance refs/iterator.c:13 #13 0x55f23e2d996e in do_for_each_ref_iterator refs/iterator.c:452 #14 0x55f23dca6767 in get_ref_map builtin/fetch.c:623 #15 0x55f23dca6767 in do_fetch builtin/fetch.c:1659 #16 0x55f23dca6767 in fetch_one builtin/fetch.c:2133 #17 0x55f23dca6767 in cmd_fetch builtin/fetch.c:2432 #18 0x55f23dba7764 in run_builtin git.c:484 #19 0x55f23dba7764 in handle_builtin git.c:741 #20 0x55f23dbab61e in run_argv git.c:805 #21 0x55f23dbab61e in cmd_main git.c:1000 #22 0x55f23dba4781 in main common-main.c:64 #23 0x7fa0f063fc89 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58 #24 0x7fa0f063fd44 in __libc_start_main_impl ../csu/libc-start.c:360 #25 0x55f23dba6ad0 in _start (git+0xadfad0) (BuildId: 803b2b7f59beb03d7849fb8294a8e2145dd4aa27) While it is somewhat awkward that the maintenance processes survive tests in the first place, it is totally expected that reftables should work alright with concurrent writers. Seemingly they don't. The only underlying resource that we need to care about in this context is the reftable reader, which is responsible for reading a single table from disk. These readers get discarded immediately (unless reused) when calling `reftable_stack_reload()`, which is wrong. We can only close them once we know that there are no iterators using them anymore. Prepare for a fix by converting the reftable readers to be refcounted. Reported-by: Jeff King Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano --- reftable/reader.c | 16 ++++++++++++++-- reftable/reader.h | 2 ++ reftable/readwrite_test.c | 18 +++++++++--------- reftable/reftable-reader.h | 15 ++++++++++++--- reftable/stack.c | 8 ++++---- reftable/stack_test.c | 6 ++---- t/helper/test-reftable.c | 2 +- t/unit-tests/t-reftable-merged.c | 4 ++-- 8 files changed, 46 insertions(+), 25 deletions(-) (limited to 'reftable/stack_test.c') diff --git a/reftable/reader.c b/reftable/reader.c index 037417fcf6..64a0953e68 100644 --- a/reftable/reader.c +++ b/reftable/reader.c @@ -621,6 +621,7 @@ int reftable_reader_new(struct reftable_reader **out, r->source = *source; r->name = xstrdup(name); r->hash_id = 0; + r->refcount = 1; err = block_source_read_block(source, &footer, r->size, footer_size(r->version)); @@ -645,10 +646,21 @@ int reftable_reader_new(struct reftable_reader **out, return err; } -void reftable_reader_free(struct reftable_reader *r) +void reftable_reader_incref(struct reftable_reader *r) +{ + if (!r->refcount) + BUG("cannot increment ref counter of dead reader"); + r->refcount++; +} + +void reftable_reader_decref(struct reftable_reader *r) { if (!r) return; + if (!r->refcount) + BUG("cannot decrement ref counter of dead reader"); + if (--r->refcount) + return; block_source_close(&r->source); FREE_AND_NULL(r->name); reftable_free(r); @@ -812,7 +824,7 @@ int reftable_reader_print_blocks(const char *tablename) } done: - reftable_reader_free(r); + reftable_reader_decref(r); table_iter_close(&ti); return err; } diff --git a/reftable/reader.h b/reftable/reader.h index 88b4f3b421..3710ee09b4 100644 --- a/reftable/reader.h +++ b/reftable/reader.h @@ -50,6 +50,8 @@ struct reftable_reader { struct reftable_reader_offsets ref_offsets; struct reftable_reader_offsets obj_offsets; struct reftable_reader_offsets log_offsets; + + uint64_t refcount; }; const char *reader_name(struct reftable_reader *r); diff --git a/reftable/readwrite_test.c b/reftable/readwrite_test.c index 2f2ff787b2..0494e7955a 100644 --- a/reftable/readwrite_test.c +++ b/reftable/readwrite_test.c @@ -279,7 +279,7 @@ static void test_log_write_read(void) /* cleanup. */ strbuf_release(&buf); free_names(names); - reftable_reader_free(reader); + reftable_reader_decref(reader); } static void test_log_zlib_corruption(void) @@ -341,7 +341,7 @@ static void test_log_zlib_corruption(void) reftable_iterator_destroy(&it); /* cleanup. */ - reftable_reader_free(reader); + reftable_reader_decref(reader); strbuf_release(&buf); } @@ -383,7 +383,7 @@ static void test_table_read_write_sequential(void) EXPECT(j == N); reftable_iterator_destroy(&it); - reftable_reader_free(reader); + reftable_reader_decref(reader); strbuf_release(&buf); free_names(names); } @@ -431,7 +431,7 @@ static void test_table_read_api(void) } reftable_iterator_destroy(&it); reftable_free(names); - reftable_reader_free(reader); + reftable_reader_decref(reader); strbuf_release(&buf); } @@ -498,7 +498,7 @@ static void test_table_read_write_seek(int index, int hash_id) reftable_free(names[i]); } reftable_free(names); - reftable_reader_free(reader); + reftable_reader_decref(reader); } static void test_table_read_write_seek_linear(void) @@ -611,7 +611,7 @@ static void test_table_refs_for(int indexed) strbuf_release(&buf); free_names(want_names); reftable_iterator_destroy(&it); - reftable_reader_free(reader); + reftable_reader_decref(reader); } static void test_table_refs_for_no_index(void) @@ -657,7 +657,7 @@ static void test_write_empty_table(void) EXPECT(err > 0); reftable_iterator_destroy(&it); - reftable_reader_free(rd); + reftable_reader_decref(rd); strbuf_release(&buf); } @@ -863,7 +863,7 @@ static void test_write_multiple_indices(void) reftable_iterator_destroy(&it); reftable_writer_free(writer); - reftable_reader_free(reader); + reftable_reader_decref(reader); strbuf_release(&writer_buf); strbuf_release(&buf); } @@ -919,7 +919,7 @@ static void test_write_multi_level_index(void) reftable_iterator_destroy(&it); reftable_writer_free(writer); - reftable_reader_free(reader); + reftable_reader_decref(reader); strbuf_release(&writer_buf); strbuf_release(&buf); } diff --git a/reftable/reftable-reader.h b/reftable/reftable-reader.h index 8a05c84685..a600452b56 100644 --- a/reftable/reftable-reader.h +++ b/reftable/reftable-reader.h @@ -33,6 +33,18 @@ struct reftable_reader; int reftable_reader_new(struct reftable_reader **pp, struct reftable_block_source *src, const char *name); +/* + * Manage the reference count of the reftable reader. A newly initialized + * reader starts with a refcount of 1 and will be deleted once the refcount has + * reached 0. + * + * This is required because readers may have longer lifetimes than the stack + * they belong to. The stack may for example be reloaded while the old tables + * are still being accessed by an iterator. + */ +void reftable_reader_incref(struct reftable_reader *reader); +void reftable_reader_decref(struct reftable_reader *reader); + /* Initialize a reftable iterator for reading refs. */ void reftable_reader_init_ref_iterator(struct reftable_reader *r, struct reftable_iterator *it); @@ -44,9 +56,6 @@ void reftable_reader_init_log_iterator(struct reftable_reader *r, /* returns the hash ID used in this table. */ uint32_t reftable_reader_hash_id(struct reftable_reader *r); -/* closes and deallocates a reader. */ -void reftable_reader_free(struct reftable_reader *); - /* return an iterator for the refs pointing to `oid`. */ int reftable_reader_refs_for(struct reftable_reader *r, struct reftable_iterator *it, uint8_t *oid); diff --git a/reftable/stack.c b/reftable/stack.c index 0ac9cdf8de..8e85f8b4d9 100644 --- a/reftable/stack.c +++ b/reftable/stack.c @@ -186,7 +186,7 @@ void reftable_stack_destroy(struct reftable_stack *st) if (names && !has_name(names, name)) { stack_filename(&filename, st, name); } - reftable_reader_free(st->readers[i]); + reftable_reader_decref(st->readers[i]); if (filename.len) { /* On Windows, can only unlink after closing. */ @@ -290,7 +290,7 @@ static int reftable_stack_reload_once(struct reftable_stack *st, const char *name = reader_name(cur[i]); stack_filename(&table_path, st, name); - reftable_reader_free(cur[i]); + reftable_reader_decref(cur[i]); /* On Windows, can only unlink after closing. */ unlink(table_path.buf); @@ -299,7 +299,7 @@ static int reftable_stack_reload_once(struct reftable_stack *st, done: for (i = 0; i < new_readers_len; i++) - reftable_reader_free(new_readers[i]); + reftable_reader_decref(new_readers[i]); reftable_free(new_readers); reftable_free(cur); strbuf_release(&table_path); @@ -1534,7 +1534,7 @@ static void remove_maybe_stale_table(struct reftable_stack *st, uint64_t max, goto done; update_idx = reftable_reader_max_update_index(rd); - reftable_reader_free(rd); + reftable_reader_decref(rd); if (update_idx <= max) { unlink(table_path.buf); diff --git a/reftable/stack_test.c b/reftable/stack_test.c index de0669b7b8..bc3bf77274 100644 --- a/reftable/stack_test.c +++ b/reftable/stack_test.c @@ -1036,10 +1036,8 @@ static void test_reftable_stack_compaction_concurrent(void) static void unclean_stack_close(struct reftable_stack *st) { /* break abstraction boundary to simulate unclean shutdown. */ - int i = 0; - for (; i < st->readers_len; i++) { - reftable_reader_free(st->readers[i]); - } + for (size_t i = 0; i < st->readers_len; i++) + reftable_reader_decref(st->readers[i]); st->readers_len = 0; FREE_AND_NULL(st->readers); } diff --git a/t/helper/test-reftable.c b/t/helper/test-reftable.c index 87c2f42aae..f6d855a9d7 100644 --- a/t/helper/test-reftable.c +++ b/t/helper/test-reftable.c @@ -152,7 +152,7 @@ static int dump_reftable(const char *tablename) done: reftable_merged_table_free(mt); - reftable_reader_free(r); + reftable_reader_decref(r); return err; } diff --git a/t/unit-tests/t-reftable-merged.c b/t/unit-tests/t-reftable-merged.c index 8f51aca1b6..081d3c8b69 100644 --- a/t/unit-tests/t-reftable-merged.c +++ b/t/unit-tests/t-reftable-merged.c @@ -115,7 +115,7 @@ merged_table_from_records(struct reftable_ref_record **refs, static void readers_destroy(struct reftable_reader **readers, const size_t n) { for (size_t i = 0; i < n; i++) - reftable_reader_free(readers[i]); + reftable_reader_decref(readers[i]); reftable_free(readers); } @@ -437,7 +437,7 @@ static void t_default_write_opts(void) err = reftable_merged_table_new(&merged, &rd, 1, GIT_SHA1_FORMAT_ID); check(!err); - reftable_reader_free(rd); + reftable_reader_decref(rd); reftable_merged_table_free(merged); strbuf_release(&buf); } -- cgit 1.2.3-korg From 89eada4ea1f7a9c0a5f9b9e29592daa0847a79fc Mon Sep 17 00:00:00 2001 From: Patrick Steinhardt Date: Fri, 23 Aug 2024 16:12:51 +0200 Subject: reftable/reader: keep readers alive during iteration The lifetime of a table iterator may survive the lifetime of a reader when the stack gets reloaded. Keep the reader from being released by increasing its refcount while the iterator is still being used. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano --- reftable/reader.c | 2 ++ reftable/stack_test.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 52 insertions(+) (limited to 'reftable/stack_test.c') diff --git a/reftable/reader.c b/reftable/reader.c index 64a0953e68..f877099087 100644 --- a/reftable/reader.c +++ b/reftable/reader.c @@ -175,6 +175,7 @@ static int table_iter_init(struct table_iter *ti, struct reftable_reader *r) { struct block_iter bi = BLOCK_ITER_INIT; memset(ti, 0, sizeof(*ti)); + reftable_reader_incref(r); ti->r = r; ti->bi = bi; return 0; @@ -262,6 +263,7 @@ static void table_iter_close(struct table_iter *ti) { table_iter_block_done(ti); block_iter_close(&ti->bi); + reftable_reader_decref(ti->r); } static int table_iter_next_block(struct table_iter *ti) diff --git a/reftable/stack_test.c b/reftable/stack_test.c index bc3bf77274..7fb5beb7c9 100644 --- a/reftable/stack_test.c +++ b/reftable/stack_test.c @@ -1076,6 +1076,55 @@ static void test_reftable_stack_compaction_concurrent_clean(void) clear_dir(dir); } +static void test_reftable_stack_read_across_reload(void) +{ + struct reftable_write_options opts = { 0 }; + struct reftable_stack *st1 = NULL, *st2 = NULL; + struct reftable_ref_record rec = { 0 }; + struct reftable_iterator it = { 0 }; + char *dir = get_tmp_dir(__LINE__); + int err; + + /* Create a first stack and set up an iterator for it. */ + err = reftable_new_stack(&st1, dir, &opts); + EXPECT_ERR(err); + write_n_ref_tables(st1, 2); + EXPECT(st1->merged->readers_len == 2); + reftable_stack_init_ref_iterator(st1, &it); + err = reftable_iterator_seek_ref(&it, ""); + EXPECT_ERR(err); + + /* Set up a second stack for the same directory and compact it. */ + err = reftable_new_stack(&st2, dir, &opts); + EXPECT_ERR(err); + EXPECT(st2->merged->readers_len == 2); + err = reftable_stack_compact_all(st2, NULL); + EXPECT_ERR(err); + EXPECT(st2->merged->readers_len == 1); + + /* + * Verify that we can continue to use the old iterator even after we + * have reloaded its stack. + */ + err = reftable_stack_reload(st1); + EXPECT_ERR(err); + EXPECT(st1->merged->readers_len == 1); + err = reftable_iterator_next_ref(&it, &rec); + EXPECT_ERR(err); + EXPECT(!strcmp(rec.refname, "refs/heads/branch-0000")); + err = reftable_iterator_next_ref(&it, &rec); + EXPECT_ERR(err); + EXPECT(!strcmp(rec.refname, "refs/heads/branch-0001")); + err = reftable_iterator_next_ref(&it, &rec); + EXPECT(err > 0); + + reftable_ref_record_release(&rec); + reftable_iterator_destroy(&it); + reftable_stack_destroy(st1); + reftable_stack_destroy(st2); + clear_dir(dir); +} + int stack_test_main(int argc, const char *argv[]) { RUN_TEST(test_empty_add); @@ -1098,6 +1147,7 @@ int stack_test_main(int argc, const char *argv[]) RUN_TEST(test_reftable_stack_auto_compaction_fails_gracefully); RUN_TEST(test_reftable_stack_update_index_check); RUN_TEST(test_reftable_stack_uptodate); + RUN_TEST(test_reftable_stack_read_across_reload); RUN_TEST(test_suggest_compaction_segment); RUN_TEST(test_suggest_compaction_segment_nothing); return 0; -- cgit 1.2.3-korg From 85da2a2ab62e24a8b9ff183fe3a451b445632487 Mon Sep 17 00:00:00 2001 From: Patrick Steinhardt Date: Fri, 23 Aug 2024 16:12:57 +0200 Subject: reftable/stack: fix segfault when reload with reused readers fails It is expected that reloading the stack fails with concurrent writers, e.g. because a table that we just wanted to read just got compacted. In case we decided to reuse readers this will cause a segfault though because we unconditionally release all new readers, including the reused ones. As those are still referenced by the current stack, the result is that we will eventually try to dereference those already-freed readers. Fix this bug by incrementing the refcount of reused readers temporarily. Signed-off-by: Patrick Steinhardt Signed-off-by: Junio C Hamano --- reftable/stack.c | 23 ++++++++++++++++++++ reftable/stack_test.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 82 insertions(+) (limited to 'reftable/stack_test.c') diff --git a/reftable/stack.c b/reftable/stack.c index 0247222258..ce0a35216b 100644 --- a/reftable/stack.c +++ b/reftable/stack.c @@ -226,6 +226,8 @@ static int reftable_stack_reload_once(struct reftable_stack *st, { size_t cur_len = !st->merged ? 0 : st->merged->readers_len; struct reftable_reader **cur = stack_copy_readers(st, cur_len); + struct reftable_reader **reused = NULL; + size_t reused_len = 0, reused_alloc = 0; size_t names_len = names_length(names); struct reftable_reader **new_readers = reftable_calloc(names_len, sizeof(*new_readers)); @@ -245,6 +247,18 @@ static int reftable_stack_reload_once(struct reftable_stack *st, if (cur[i] && 0 == strcmp(cur[i]->name, name)) { rd = cur[i]; cur[i] = NULL; + + /* + * When reloading the stack fails, we end up + * releasing all new readers. This also + * includes the reused readers, even though + * they are still in used by the old stack. We + * thus need to keep them alive here, which we + * do by bumping their refcount. + */ + REFTABLE_ALLOC_GROW(reused, reused_len + 1, reused_alloc); + reused[reused_len++] = rd; + reftable_reader_incref(rd); break; } } @@ -301,10 +315,19 @@ static int reftable_stack_reload_once(struct reftable_stack *st, new_readers = NULL; new_readers_len = 0; + /* + * Decrement the refcount of reused readers again. This only needs to + * happen on the successful case, because on the unsuccessful one we + * decrement their refcount via `new_readers`. + */ + for (i = 0; i < reused_len; i++) + reftable_reader_decref(reused[i]); + done: for (i = 0; i < new_readers_len; i++) reftable_reader_decref(new_readers[i]); reftable_free(new_readers); + reftable_free(reused); reftable_free(cur); strbuf_release(&table_path); return err; diff --git a/reftable/stack_test.c b/reftable/stack_test.c index 7fb5beb7c9..6809bf9d30 100644 --- a/reftable/stack_test.c +++ b/reftable/stack_test.c @@ -10,6 +10,7 @@ license that can be found in the LICENSE file or at #include "system.h" +#include "copy.h" #include "reftable-reader.h" #include "merged.h" #include "basics.h" @@ -1125,6 +1126,63 @@ static void test_reftable_stack_read_across_reload(void) clear_dir(dir); } +static void test_reftable_stack_reload_with_missing_table(void) +{ + struct reftable_write_options opts = { 0 }; + struct reftable_stack *st = NULL; + struct reftable_ref_record rec = { 0 }; + struct reftable_iterator it = { 0 }; + struct strbuf table_path = STRBUF_INIT, content = STRBUF_INIT; + char *dir = get_tmp_dir(__LINE__); + int err; + + /* Create a first stack and set up an iterator for it. */ + err = reftable_new_stack(&st, dir, &opts); + EXPECT_ERR(err); + write_n_ref_tables(st, 2); + EXPECT(st->merged->readers_len == 2); + reftable_stack_init_ref_iterator(st, &it); + err = reftable_iterator_seek_ref(&it, ""); + EXPECT_ERR(err); + + /* + * Update the tables.list file with some garbage data, while reusing + * our old readers. This should trigger a partial reload of the stack, + * where we try to reuse our old readers. + */ + strbuf_addf(&content, "%s\n", st->readers[0]->name); + strbuf_addf(&content, "%s\n", st->readers[1]->name); + strbuf_addstr(&content, "garbage\n"); + strbuf_addf(&table_path, "%s.lock", st->list_file); + write_file_buf(table_path.buf, content.buf, content.len); + err = rename(table_path.buf, st->list_file); + EXPECT_ERR(err); + + err = reftable_stack_reload(st); + EXPECT(err == -4); + EXPECT(st->merged->readers_len == 2); + + /* + * Even though the reload has failed, we should be able to continue + * using the iterator. + */ + err = reftable_iterator_next_ref(&it, &rec); + EXPECT_ERR(err); + EXPECT(!strcmp(rec.refname, "refs/heads/branch-0000")); + err = reftable_iterator_next_ref(&it, &rec); + EXPECT_ERR(err); + EXPECT(!strcmp(rec.refname, "refs/heads/branch-0001")); + err = reftable_iterator_next_ref(&it, &rec); + EXPECT(err > 0); + + reftable_ref_record_release(&rec); + reftable_iterator_destroy(&it); + reftable_stack_destroy(st); + strbuf_release(&table_path); + strbuf_release(&content); + clear_dir(dir); +} + int stack_test_main(int argc, const char *argv[]) { RUN_TEST(test_empty_add); @@ -1148,6 +1206,7 @@ int stack_test_main(int argc, const char *argv[]) RUN_TEST(test_reftable_stack_update_index_check); RUN_TEST(test_reftable_stack_uptodate); RUN_TEST(test_reftable_stack_read_across_reload); + RUN_TEST(test_reftable_stack_reload_with_missing_table); RUN_TEST(test_suggest_compaction_segment); RUN_TEST(test_suggest_compaction_segment_nothing); return 0; -- cgit 1.2.3-korg