# Segmentation fault within git read-tree 4 messages from 2023-01-14 to 2023-01-15. Participants: Frédéric Fort, Christian Couder, Jeff King, Junio C Hamano. Thread: https://gitlist.dev/t/59092 ## Frédéric Fort, 2023-01-14 22:14 Subject: Segmentation fault within git read-tree Message-ID: URL: https://gitlist.dev/e/d0f9520d-9ccc-a899-609e-fbbb4529e005%40free.fr ``` Hello, I am doing some tests trying to do a sparse checkout of a partial clone within a subtree. However, I get a segfault when trying to run git read-tree as is done by git subtree internally. Maybe what I am doing isn't supposed to work at all, but I suppose it shouldn't at least cause git read-tree to segfault. Here should be a MWE to reproduce my error: # Run this to create the repo that will become your subtree git init subtree.git cd subtree.git touch x y git add . git commit -m "first commit" # Run this to create the repo that has a sparse checkout of a partial clone within a subtree git init repo cd repo git commit --allow-empty "first commit" git sparse-checkout set "subtree/x" git remote add origin-subtree git@my.server:/with/the/subtree.git git fetch --filter=blob:none origin-subtree git rev-parse --verify "FETCH_HEAD^{commit}" git read-tree --debug-unpack --prefix="subtree" FETCH_HEAD Yours sincerely, Frédéric Fort ``` ## Christian Couder, 2023-01-15 11:53 Subject: Re: Segmentation fault within git read-tree Message-ID: URL: https://gitlist.dev/e/CAP8UFD0pKZT57jpUOg7gckcr4stoq24YDB2Fu0-AwbGPrEweqg%40mail.gmail.com In-Reply-To: ``` Hi, On Sat, Jan 14, 2023 at 11:22 PM Frédéric Fort wrote: > > Hello, > > I am doing some tests trying to do a sparse checkout of a partial clone > within a subtree. > However, I get a segfault when trying to run git read-tree as is done by > git subtree internally. > > Maybe what I am doing isn't supposed to work at all, but I suppose it > shouldn't at least cause git read-tree to segfault. Yeah, it shouldn't segfault. Thanks for the report! > Here should be a MWE to reproduce my error: I reproduced the issue using your steps with a few changes. > # Run this to create the repo that will become your subtree > git init subtree.git > cd subtree.git > touch x y > git add . > git commit -m "first commit" > > # Run this to create the repo that has a sparse checkout of a partial > clone within a subtree > git init repo > cd repo > git commit --allow-empty "first commit" I think the above command is missing '-m' before "first commit". > git sparse-checkout set "subtree/x" > git remote add origin-subtree git@my.server:/with/the/subtree.git I reproduced using a local remote like: git remote add origin-subtree /path/to/subtree.git > git fetch --filter=blob:none origin-subtree > git rev-parse --verify "FETCH_HEAD^{commit}" > git read-tree --debug-unpack --prefix="subtree" FETCH_HEAD Yeah, I get the following under gdb: Program received signal SIGSEGV, Segmentation fault. debug_path (info=0x7fffffffc880) at unpack-trees.c:1395 1395 if (*info->prev->name) (gdb) bt #0 debug_path (info=0x7fffffffc880) at unpack-trees.c:1395 #1 0x000055555587d917 in debug_unpack_callback (n=1, mask=1, dirmask=0, names=0x7fffffffc3b0, info=0x7fffffffc880) at unpack-trees.c:1417 #2 0x000055555587dc48 in unpack_callback (n=1, mask=1, dirmask=0, names=0x7fffffffc3b0, info=0x7fffffffc880) at unpack-trees.c:1491 #3 0x00005555558779c2 in traverse_trees (istate=0x5555559f3b00 , n=1, t=0x7fffffffcb50, info=0x7fffffffc880) at tree-walk.c:536 #4 0x000055555587ef85 in unpack_trees (len=1, t=0x7fffffffcb50, o=0x7fffffffcd90) at unpack-trees.c:1979 #5 0x000055555562ab45 in cmd_read_tree (argc=1, argv=0x7fffffffdc90, cmd_prefix=0x0) at builtin/read-tree.c:263 #6 0x0000555555575a4d in run_builtin (p=0x5555559bf8f0 , argc=4, argv=0x7fffffffdc90) at git.c:445 #7 0x0000555555575ed2 in handle_builtin (argc=4, argv=0x7fffffffdc90) at git.c:700 #8 0x0000555555576179 in run_argv (argcp=0x7fffffffdaec, argv=0x7fffffffdae0) at git.c:764 #9 0x0000555555576764 in cmd_main (argc=4, argv=0x7fffffffdc90) at git.c:899 #10 0x0000555555682dfe in main (argc=5, argv=0x7fffffffdc88) at common-main.c:57 From a very quick look, it seems that in setup_traverse_info() in tree-walk.c we do: static struct traverse_info dummy; ... if (pathlen) info->prev = &dummy; but then later in debug_path() in unpack-trees.c we check *info->prev->name which segfaults. I am not very familiar with this code and don't have much time right now, so I think I will leave it to others to investigate this further. Best, Christian. ``` ## Jeff King, 2023-01-15 18:35 Subject: Re: Segmentation fault within git read-tree Message-ID: URL: https://gitlist.dev/e/Y8RHfjFFLRdW3WRh%40coredump.intra.peff.net In-Reply-To: ``` On Sun, Jan 15, 2023 at 12:53:25PM +0100, Christian Couder wrote: > From a very quick look, it seems that in setup_traverse_info() in > tree-walk.c we do: > > static struct traverse_info dummy; > ... > if (pathlen) > info->prev = &dummy; > > but then later in debug_path() in unpack-trees.c we check > *info->prev->name which segfaults. > > I am not very familiar with this code and don't have much time right > now, so I think I will leave it to others to investigate this further. I'm not sure if Frédéric is seeing another segfault in practice (when not using --debug-unpack), but yeah, it is very easy to trigger this segfault. It does not even have to do with sparse checkouts, etc. Here's an even more minimal example: git init repo cd repo touch file git add file git commit -m added git read-tree --debug-unpack --prefix=subtree HEAD I was going to bisect, but it looks like it was broken all the way back to Junio's ba655da537 (read-tree --debug-unpack, 2009-09-14). -Peff ``` ## Junio C Hamano, 2023-01-15 21:49 Subject: Re: Segmentation fault within git read-tree Message-ID: URL: https://gitlist.dev/e/xmqqedrvfoyz.fsf%40gitster.g In-Reply-To: ``` Jeff King writes: > I'm not sure if Frédéric is seeing another segfault in practice (when > not using --debug-unpack), but yeah, it is very easy to trigger this > segfault. It does not even have to do with sparse checkouts, etc. Here's > an even more minimal example: > > git init repo > cd repo > touch file > git add file > git commit -m added > git read-tree --debug-unpack --prefix=subtree HEAD > > I was going to bisect, but it looks like it was broken all the way back > to Junio's ba655da537 (read-tree --debug-unpack, 2009-09-14). As "git read-tree --help" does not even metnion "--debug-unpack", I have no idea what it does, or how useful the debug information it produces is. As long as the same command without --debug-unpack works OK, I'd throw this into "does it hurt? don't do it, then" bin. ;-) ```