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firoj0
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Dec 3, 2024
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
git#6 0x55f23e5573dc in table_iter_next reftable/reader.c:339
#7 0x55f23e551283 in merged_iter_advance_subiter reftable/merged.c:69
git#8 0x55f23e55169e in merged_iter_next_entry reftable/merged.c:123
git#9 0x55f23e55169e in merged_iter_next_void reftable/merged.c:172
git#10 0x55f23e537625 in reftable_iterator_next_ref reftable/generic.c:175
git#11 0x55f23e2cf9c6 in reftable_ref_iterator_advance refs/reftable-backend.c:464
git#12 0x55f23e2d996e in ref_iterator_advance refs/iterator.c:13
git#13 0x55f23e2d996e in do_for_each_ref_iterator refs/iterator.c:452
git#14 0x55f23dca6767 in get_ref_map builtin/fetch.c:623
git#15 0x55f23dca6767 in do_fetch builtin/fetch.c:1659
git#16 0x55f23dca6767 in fetch_one builtin/fetch.c:2133
git#17 0x55f23dca6767 in cmd_fetch builtin/fetch.c:2432
git#18 0x55f23dba7764 in run_builtin git.c:484
git#19 0x55f23dba7764 in handle_builtin git.c:741
git#20 0x55f23dbab61e in run_argv git.c:805
git#21 0x55f23dbab61e in cmd_main git.c:1000
git#22 0x55f23dba4781 in main common-main.c:64
git#23 0x7fa0f063fc89 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
git#24 0x7fa0f063fd44 in __libc_start_main_impl ../csu/libc-start.c:360
git#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 <[email protected]>
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
firoj0
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Dec 3, 2024
This one is a little bit more curious. In t6112, we have a test that
exercises the `git rev-list --filter` option with invalid filters. We
execute git-rev-list(1) via `test_must_fail`, which means that we check
for leaks even though Git exits with an error code. This causes the
following leak:
Direct leak of 27 byte(s) in 1 object(s) allocated from:
#0 0x5555555e6946 in realloc.part.0 lsan_interceptors.cpp.o
#1 0x5555558fb4b6 in xrealloc wrapper.c:137:8
#2 0x5555558b6e06 in strbuf_grow strbuf.c:112:2
#3 0x5555558b7550 in strbuf_add strbuf.c:311:2
#4 0x5555557c1a88 in strbuf_addstr strbuf.h:310:2
#5 0x5555557c1d4c in parse_list_objects_filter list-objects-filter-options.c:261:3
git#6 0x555555885ead in handle_revision_pseudo_opt revision.c:2899:3
#7 0x555555884e20 in setup_revisions revision.c:3014:11
git#8 0x5555556c4b42 in cmd_rev_list builtin/rev-list.c:588:9
git#9 0x5555555ec5e3 in run_builtin git.c:483:11
git#10 0x5555555eb1e4 in handle_builtin git.c:749:13
git#11 0x5555555ec001 in run_argv git.c:819:4
git#12 0x5555555eaf94 in cmd_main git.c:954:19
git#13 0x5555556fd569 in main common-main.c:64:11
git#14 0x7ffff7ca714d in __libc_start_call_main (.../lib/libc.so.6+0x2a14d)
git#15 0x7ffff7ca7208 in __libc_start_main@GLIBC_2.2.5 (.../libc.so.6+0x2a208)
git#16 0x5555555ad064 in _start (git+0x59064)
This leak is valid, as we call `die()` and do not clean up the memory at
all. But what's curious is that this is the only leak reported, because
we don't clean up any other allocated memory, either, and I have no idea
why the leak sanitizer treats this buffer specially.
In any case, we can work around the leak by shuffling things around a
bit. Instead of calling `gently_parse_list_objects_filter()` and dying
after we have modified the filter spec, we simply do so beforehand. Like
this we don't allocate the buffer in the error case, which makes the
reported leak go away.
It's not pretty, but it manages to make t6112 leak free.
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
firoj0
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Dec 30, 2024
When running t5601 with the leak checker enabled we can see a hang in
our CI systems. This hang seems to be system-specific, as I cannot
reproduce it on my own machine.
As it turns out, the issue is in those testcases that exercise cloning
of `~repo`-style paths. All of the testcases that hang eventually end up
interpreting "repo" as the username and will call getpwnam(3p) with that
username. That should of course be fine, and getpwnam(3p) should just
return an error. But instead, the leak sanitizer seems to be recursing
while handling a call to `free()` in the NSS modules:
#0 0x00007ffff7fd98d5 in _dl_update_slotinfo (req_modid=1, new_gen=2) at ../elf/dl-tls.c:720
#1 0x00007ffff7fd9ac4 in update_get_addr (ti=0x7ffff7a91d80, gen=<optimized out>) at ../elf/dl-tls.c:916
#2 0x00007ffff7fdc85c in __tls_get_addr () at ../sysdeps/x86_64/tls_get_addr.S:55
#3 0x00007ffff7a27e04 in __lsan::GetAllocatorCache () at ../../../../src/libsanitizer/lsan/lsan_linux.cpp:27
#4 0x00007ffff7a2b33a in __lsan::Deallocate (p=0x0) at ../../../../src/libsanitizer/lsan/lsan_allocator.cpp:127
#5 __lsan::lsan_free (p=0x0) at ../../../../src/libsanitizer/lsan/lsan_allocator.cpp:220
...
#261505 0x00007ffff7fd99f2 in free (ptr=<optimized out>) at ../include/rtld-malloc.h:50
#261506 _dl_update_slotinfo (req_modid=1, new_gen=2) at ../elf/dl-tls.c:822
#261507 0x00007ffff7fd9ac4 in update_get_addr (ti=0x7ffff7a91d80, gen=<optimized out>) at ../elf/dl-tls.c:916
#261508 0x00007ffff7fdc85c in __tls_get_addr () at ../sysdeps/x86_64/tls_get_addr.S:55
#261509 0x00007ffff7a27e04 in __lsan::GetAllocatorCache () at ../../../../src/libsanitizer/lsan/lsan_linux.cpp:27
#261510 0x00007ffff7a2b33a in __lsan::Deallocate (p=0x5020000001e0) at ../../../../src/libsanitizer/lsan/lsan_allocator.cpp:127
#261511 __lsan::lsan_free (p=0x5020000001e0) at ../../../../src/libsanitizer/lsan/lsan_allocator.cpp:220
#261512 0x00007ffff793da25 in module_load (module=0x515000000280) at ./nss/nss_module.c:188
#261513 0x00007ffff793dee5 in __nss_module_load (module=0x515000000280) at ./nss/nss_module.c:302
#261514 __nss_module_get_function (module=0x515000000280, name=name@entry=0x7ffff79b9128 "getpwnam_r") at ./nss/nss_module.c:328
#261515 0x00007ffff793e741 in __GI___nss_lookup_function (fct_name=<optimized out>, ni=<optimized out>) at ./nss/nsswitch.c:137
#261516 __GI___nss_next2 (ni=ni@entry=0x7fffffffa458, fct_name=fct_name@entry=0x7ffff79b9128 "getpwnam_r", fct2_name=fct2_name@entry=0x0, fctp=fctp@entry=0x7fffffffa460,
status=status@entry=0, all_values=all_values@entry=0) at ./nss/nsswitch.c:120
#261517 0x00007ffff794c6a7 in __getpwnam_r (name=name@entry=0x501000000060 "repo", resbuf=resbuf@entry=0x7ffff79fb320 <resbuf>, buffer=<optimized out>,
buflen=buflen@entry=1024, result=result@entry=0x7fffffffa4b0) at ../nss/getXXbyYY_r.c:343
#261518 0x00007ffff794c4d8 in getpwnam (name=0x501000000060 "repo") at ../nss/getXXbyYY.c:140
#261519 0x00005555557e37ff in getpw_str (username=0x5020000001a1 "repo", len=4) at path.c:613
#261520 0x00005555557e3937 in interpolate_path (path=0x5020000001a0 "~repo", real_home=0) at path.c:654
#261521 0x00005555557e3aea in enter_repo (path=0x501000000040 "~repo", strict=0) at path.c:718
#261522 0x000055555568f0ba in cmd_upload_pack (argc=1, argv=0x502000000100, prefix=0x0, repo=0x0) at builtin/upload-pack.c:57
#261523 0x0000555555575ba8 in run_builtin (p=0x555555a20c98 <commands+3192>, argc=2, argv=0x502000000100, repo=0x555555a53b20 <the_repo>) at git.c:481
#261524 0x0000555555576067 in handle_builtin (args=0x7fffffffaab0) at git.c:742
#261525 0x000055555557678d in cmd_main (argc=2, argv=0x7fffffffac58) at git.c:912
#261526 0x00005555556963cd in main (argc=2, argv=0x7fffffffac58) at common-main.c:64
Note that this stack is more than 260000 function calls deep. Run under
the debugger this will eventually segfault, but in our CI systems it
seems like this just hangs forever.
I assume that this is a bug either in the leak sanitizer or in glibc, as
I cannot reproduce it on my machine. In any case, let's work around the
bug for now by marking those tests with the "!SANITIZE_LEAK" prereq.
Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
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