Re: [BUG] rebase: can write reflog with uninit. `action` string

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Kristoffer

On 28/04/2025 20:40, Kristoffer Haugsbakk wrote:
> From: code@khaugsbakk.name
> 
> We did `git rebase --rebase-merges` on a branch with merge commits.  Including
> back merges.  The reflog after that showed some weird symbols for
> certain merge commits (only merges):
> 
>      e9c962f2ea0 HEAD@{8}: <binary>�: Merged in <branch> (pull request #4441)

Thanks for reporting this. I'm afraid I'm rather confused. In 
run_git_commit() we set GIT_REFLOG_ACTION with

	strvec_pushf(&cmd.env, GIT_REFLOG_ACTION "=%s", ctx->reflog_message);

ctx is allocated with calloc() so ctx->reflog_message is initially NULL. 
At the start of pick_commits() we store the result of 
sequencer_reflog_action() in ctx->reflog_message. That function returns 
a heap allocation that is not freed until we call replay_opts_release() 
at the end of the rebase. If we're doing a normal pick rather than a 
merge we store the result of reflog_message() in ctx->reflog_message. 
That function returns a string stored in a static strbuf. So when we 
call do_merge() from pick_commits() ctx->reflog_message should point to 
a valid string though that string will not hold the correct reflog 
message for the merge because do_merge() does not call reflog_message().

> This is the backtrace when I get the apparently uninit. string:
> 
> ```
> #0  run_git_commit (defmsg=0x555555babe70 "<repo path>/MERGE_MSG", opts=0x7fffffffc070, flags=0) at sequencer.c:1158

Thanks for the backtrace. It would be useful to know what's stored in 
opts->ctx->reflog_message at this point if it's not too much trouble 
please can you run "print *opts" and "print *opts->ctx" here.

Thanks

Phillip

> #1  0x00005555558740d9 in do_merge (r=0x555555a67020 <the_repo>, commit=0x555555b75cb0,
>      arg=0x555555b2164d "<branch stuff> # Merged in <branch> { (pull request #4464)\nlabel branch-point-9\npick 4026b5ced849724bd3857283b6ad50c8609b6d33 only sh"..., arg_len=125, flags=0, check_todo=0x7fffffffb1e0, opts=0x7fffffffc070) at sequencer.c:4380
> #2  0x0000555555876629 in pick_commits (r=0x555555a67020 <the_repo>, todo_list=0x7fffffffbf50, opts=0x7fffffffc070) at sequencer.c:5048
> #3  0x0000555555877eeb in sequencer_continue (r=0x555555a67020 <the_repo>, opts=0x7fffffffc070) at sequencer.c:5480
> #4  0x000055555563a491 in run_sequencer_rebase (opts=0x7fffffffc330) at builtin/rebase.c:369
> #5  0x000055555563bc74 in run_specific_rebase (opts=0x7fffffffc330) at builtin/rebase.c:746
> #6  0x000055555563fe2a in cmd_rebase (argc=0, argv=0x555555a73890, prefix=0x0, repo=0x555555a67020 <the_repo>) at builtin/rebase.c:1878
> #7  0x0000555555574c0d in run_builtin (p=0x555555a34908 <commands+2280>, argc=2, argv=0x555555a73890, repo=0x555555a67020 <the_repo>) at git.c:480
> #8  0x00005555555750ca in handle_builtin (args=0x7fffffffd8a0) at git.c:743
> #9  0x000055555557538c in run_argv (args=0x7fffffffd8a0) at git.c:810
> #10 0x00005555555759e2 in cmd_main (argc=2, argv=0x7fffffffda30) at git.c:950
> #11 0x000055555569b0c3 in main (argc=5, argv=0x7fffffffda18) at common-main.c:9
> ```
> 
> § No reproduction script
> 
> I was unable to reproduce with a simple repo. setup.  I tried:
> 
> 1. Creating a side branch which had a merge
> 2. The side branch conflicted with the other branch
> 3. Rebased with `--rebase-merges`
> 
> -----
> 
> Normal bugreport questionaire follows.
> 
> -----
> 
>      Thank you for filling out a Git bug report!
>      Please answer the following questions to help us understand your issue.
> 
>      What did you do before the bug happened? (Steps to reproduce your issue)
> 
> `git rebase --rebase-merges` on a branch with merge commits.  Including
> back merges.
> 
>      What did you expect to happen? (Expected behavior)
> 
> Normal “action” string for the reflog like for example `continue`:
> 
>      3f90f6ab14d (HEAD -> <branch>) HEAD@{1}: rebase (continue): Merged in <branch> (pull request #4507)
> 
> What happened instead? (Actual behavior)
> 
> The “action” (or whatever it is) string is arbitrary bytes.  Like some
> uninit. memory.
> 
> ```
> e9c962f2ea0 HEAD@{8}: <binary>�: Merged in <branch> (pull request #4441)
> ```
> 
>      What's different between what you expected and what actually happened?
> 
> Apparently uninit. string.
> 
>       Anything else you want to add:
> 
>       Please review the rest of the bug report below.
>       You can delete any lines you don't wish to share.
> 
> 
> [System Info]
> git version:
> git version 2.49.0.459.gf65182a99e5
> cpu: x86_64
> built from commit: f65182a99e545d2f2bc22e6c1c2da192133b16a3
> sizeof-long: 8
> sizeof-size_t: 8
> shell-path: /bin/sh
> libcurl: 7.81.0
> OpenSSL: OpenSSL 3.0.2 15 Mar 2022
> zlib: 1.2.11
> SHA-1: SHA1_DC
> SHA-256: SHA256_BLK
> uname: Linux 6.8.0-58-generic #60~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Mar 28 16:09:21 UTC 2 x86_64
> compiler info: gnuc: 11.4
> libc info: glibc: 2.35
> $SHELL (typically, interactive shell): /bin/bash
> 
> 
> [Enabled Hooks]
>