# [PATCH 0/1] Safe-guard the Windows code a bit more against getenv() problems 7 messages from 2019-02-15 to 2019-02-21. Participants: Johannes Schindelin via GitGitGadget, Jeff Hostetler, Junio C Hamano, Johannes Schindelin, Jeff King. Thread: https://gitlist.dev/t/50517 ## Johannes Schindelin via GitGitGadget, 2019-02-15 15:17 Subject: [PATCH 0/1] Safe-guard the Windows code a bit more against getenv() problems Message-ID: URL: https://gitlist.dev/e/pull.127.git.gitgitgadget%40gmail.com ``` We saw a couple of getenv() cleanups, where we now immediately duplicate the return value of getenv() in order to avoid problems. However, I am really uncomfortable with the current value (30) of GETENV_MAX_RETAIN in compat/mingw.c: it strikes me as low, given that the average number of getenv() call per Git process in Git's test suite is already 40. I'd really like to increase it, even if it won't help problems where getenv() is called in a loop whose number of iteration depends on user input (like the problem fixed in 6776a84dae (diff: ensure correct lifetime of external_diff_cmd, 2019-01-11)). It will still fend off plenty of other cases, I believe. And yes, it's in my TODOs to look back over those getenv() issues after v2.21.0 (see https://github.com/git-for-windows/git/pull/2019 for my progress). Johannes Schindelin (1): mingw: safe-guard a bit more against getenv() problems compat/mingw.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) base-commit: 2d08f3deb9feb73dc8d21d75bfd367839fc1322c Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-127%2Fdscho%2Fmingw-retain-more-getenv-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-127/dscho/mingw-retain-more-getenv-v1 Pull-Request: https://github.com/gitgitgadget/git/pull/127 -- gitgitgadget ``` ## Johannes Schindelin via GitGitGadget, 2019-02-15 15:17 Subject: [PATCH 1/1] mingw: safe-guard a bit more against getenv() problems Message-ID: <53e3d159c8c80924188f57c44efd2170612f2ee5.1550243863.git.gitgitgadget@gmail.com> URL: https://gitlist.dev/e/53e3d159c8c80924188f57c44efd2170612f2ee5.1550243863.git.gitgitgadget%40gmail.com In-Reply-To: ``` From: Johannes Schindelin Running up to v2.21.0, we fixed two bugs that were made prominent by the Windows-specific change to retain copies of only the 30 latest getenv() calls' returned strings, invalidating any copies of previous getenv() calls' return values. While this really shines a light onto bugs of the form where we hold onto getenv()'s return values without copying them, it is also a real problem for users. And even if Jeff King's patches merged via 773e408881 (Merge branch 'jk/save-getenv-result', 2019-01-29) provide further work on that front, we are far from done. Just one example: on Windows, we unset environment variables when spawning new processes, which potentially invalidates strings that were previously obtained via getenv(), and therefore we have to duplicate environment values that are somehow involved in spawning new processes (e.g. GIT_MAN_VIEWER in show_man_page()). We do not have a chance to investigate, let address, all of those issues in time for v2.21.0, so let's at least help Windows users by increasing the number of getenv() calls' return values that are kept valid. The number 64 was determined by looking at the average number of getenv() calls per process in the entire test suite run on Windows (which is around 40) and then adding a bit for good measure. And it is a power of two (which would have hit yesterday's theme perfectly). Signed-off-by: Johannes Schindelin --- compat/mingw.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/compat/mingw.c b/compat/mingw.c index 4276297595..8141f77189 100644 --- a/compat/mingw.c +++ b/compat/mingw.c @@ -1632,7 +1632,7 @@ int mingw_kill(pid_t pid, int sig) */ char *mingw_getenv(const char *name) { -#define GETENV_MAX_RETAIN 30 +#define GETENV_MAX_RETAIN 64 static char *values[GETENV_MAX_RETAIN]; static int value_counter; int len_key, len_value; -- gitgitgadget ``` ## Jeff Hostetler, 2019-02-15 16:14 Subject: Re: [PATCH 1/1] mingw: safe-guard a bit more against getenv() problems Message-ID: <96793350-0991-6e8f-6ab2-15777e2071be@jeffhostetler.com> URL: https://gitlist.dev/e/96793350-0991-6e8f-6ab2-15777e2071be%40jeffhostetler.com In-Reply-To: <53e3d159c8c80924188f57c44efd2170612f2ee5.1550243863.git.gitgitgadget@gmail.com> ``` On 2/15/2019 10:17 AM, Johannes Schindelin via GitGitGadget wrote: > From: Johannes Schindelin > > Running up to v2.21.0, we fixed two bugs that were made prominent by the > Windows-specific change to retain copies of only the 30 latest getenv() > calls' returned strings, invalidating any copies of previous getenv() > calls' return values. > > While this really shines a light onto bugs of the form where we hold > onto getenv()'s return values without copying them, it is also a real > problem for users. > > And even if Jeff King's patches merged via 773e408881 (Merge branch > 'jk/save-getenv-result', 2019-01-29) provide further work on that front, > we are far from done. Just one example: on Windows, we unset environment > variables when spawning new processes, which potentially invalidates > strings that were previously obtained via getenv(), and therefore we > have to duplicate environment values that are somehow involved in > spawning new processes (e.g. GIT_MAN_VIEWER in show_man_page()). > > We do not have a chance to investigate, let address, all of those issues > in time for v2.21.0, so let's at least help Windows users by increasing > the number of getenv() calls' return values that are kept valid. The > number 64 was determined by looking at the average number of getenv() > calls per process in the entire test suite run on Windows (which is > around 40) and then adding a bit for good measure. And it is a power of > two (which would have hit yesterday's theme perfectly). > > Signed-off-by: Johannes Schindelin > --- > compat/mingw.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/compat/mingw.c b/compat/mingw.c > index 4276297595..8141f77189 100644 > --- a/compat/mingw.c > +++ b/compat/mingw.c > @@ -1632,7 +1632,7 @@ int mingw_kill(pid_t pid, int sig) > */ > char *mingw_getenv(const char *name) > { > -#define GETENV_MAX_RETAIN 30 > +#define GETENV_MAX_RETAIN 64 > static char *values[GETENV_MAX_RETAIN]; > static int value_counter; > int len_key, len_value; > Why not use a mem_pool for this? We have that code isolated and re-usable now. Have mingw_getenv() copy the string into the pool always return the pointer from within the pool. The pool automatically handles allocating new blocks as necessary. And (if we care) we can bulk free the pool before existing. Jeff ``` ## Junio C Hamano, 2019-02-15 18:25 Subject: Re: [PATCH 1/1] mingw: safe-guard a bit more against getenv() problems Message-ID: URL: https://gitlist.dev/e/xmqqh8d46h9n.fsf%40gitster-ct.c.googlers.com In-Reply-To: <96793350-0991-6e8f-6ab2-15777e2071be@jeffhostetler.com> ``` Jeff Hostetler writes: >> diff --git a/compat/mingw.c b/compat/mingw.c >> index 4276297595..8141f77189 100644 >> --- a/compat/mingw.c >> +++ b/compat/mingw.c >> @@ -1632,7 +1632,7 @@ int mingw_kill(pid_t pid, int sig) >> */ >> char *mingw_getenv(const char *name) >> { >> -#define GETENV_MAX_RETAIN 30 >> +#define GETENV_MAX_RETAIN 64 >> static char *values[GETENV_MAX_RETAIN]; >> static int value_counter; >> int len_key, len_value; >> > > Why not use a mem_pool for this? We have that code isolated > and re-usable now. Have mingw_getenv() copy the string into > the pool always return the pointer from within the pool. The > pool automatically handles allocating new blocks as necessary. > And (if we care) we can bulk free the pool before existing. Probably a good idea in the longer term. The patch as posted would do for the upcoming release, though. Thanks, both. ``` ## Johannes Schindelin, 2019-02-18 18:57 Subject: Re: [PATCH 1/1] mingw: safe-guard a bit more against getenv() problems Message-ID: URL: https://gitlist.dev/e/nycvar.QRO.7.76.6.1902181950130.45%40tvgsbejvaqbjf.bet In-Reply-To: <96793350-0991-6e8f-6ab2-15777e2071be@jeffhostetler.com> ``` Hi Jeff, On Fri, 15 Feb 2019, Jeff Hostetler wrote: > On 2/15/2019 10:17 AM, Johannes Schindelin via GitGitGadget wrote: > > From: Johannes Schindelin > > > > diff --git a/compat/mingw.c b/compat/mingw.c > > index 4276297595..8141f77189 100644 > > --- a/compat/mingw.c > > +++ b/compat/mingw.c > > @@ -1632,7 +1632,7 @@ int mingw_kill(pid_t pid, int sig) > > */ > > char *mingw_getenv(const char *name) > > { > > -#define GETENV_MAX_RETAIN 30 > > +#define GETENV_MAX_RETAIN 64 > > static char *values[GETENV_MAX_RETAIN]; > > static int value_counter; > > int len_key, len_value; > > > > Why not use a mem_pool for this? We have that code isolated > and re-usable now. Have mingw_getenv() copy the string into > the pool always return the pointer from within the pool. The > pool automatically handles allocating new blocks as necessary. > And (if we care) we can bulk free the pool before existing. The problem with the mem_pool would be that I do *not* want to accumulate tons and tons of stale copies (remember that `difftool` bug? Apparently it calls `getenv()` in a loop that scales with the number of modified files; I would hate to have copies for all of those). And all you could do to remedy this would be to blow away all of them it the mem_pool grew too much, *including* the latest `getenv()` (which we painfully keep valid, e.g. in the case of `git_committer_info()` which calls `getenv()` *three* times *inside* the parameter list of the `fmt_ident()` function call. Of course, we could use a hashmap combined with a priority queue that would cap at a given capacity, and once that is reached, culls the oldest entries until the capacity is maintained again. Taking a step back, thought, we are talking only about the Windows-specific code, which is code we happen to have *some* control over. On other platforms, we have a lot less control. Meaning: the true fix would be some sort of static analysis that ensures that getenv()'s returned values are duplicated unless it is super easy to reason that the use is safe. Ciao, Dscho ``` ## Jeff King, 2019-02-21 13:52 Subject: Re: [PATCH 1/1] mingw: safe-guard a bit more against getenv() problems Message-ID: <20190221135207.GA20899@sigill.intra.peff.net> URL: https://gitlist.dev/e/20190221135207.GA20899%40sigill.intra.peff.net In-Reply-To: <53e3d159c8c80924188f57c44efd2170612f2ee5.1550243863.git.gitgitgadget@gmail.com> ``` On Fri, Feb 15, 2019 at 07:17:45AM -0800, Johannes Schindelin via GitGitGadget wrote: > From: Johannes Schindelin > > Running up to v2.21.0, we fixed two bugs that were made prominent by the > Windows-specific change to retain copies of only the 30 latest getenv() > calls' returned strings, invalidating any copies of previous getenv() > calls' return values. > > While this really shines a light onto bugs of the form where we hold > onto getenv()'s return values without copying them, it is also a real > problem for users. > > And even if Jeff King's patches merged via 773e408881 (Merge branch > 'jk/save-getenv-result', 2019-01-29) provide further work on that front, > we are far from done. Just one example: on Windows, we unset environment > variables when spawning new processes, which potentially invalidates > strings that were previously obtained via getenv(), and therefore we > have to duplicate environment values that are somehow involved in > spawning new processes (e.g. GIT_MAN_VIEWER in show_man_page()). Belated review, as this is already in master, but: yes, I absolutely support this patch, even on top of my patches. Those were just the cases I found by poking around for a few minutes, and I'm sure there are many more. -Peff ``` ## Jeff King, 2019-02-21 13:58 Subject: Re: [PATCH 1/1] mingw: safe-guard a bit more against getenv() problems Message-ID: <20190221135805.GB20899@sigill.intra.peff.net> URL: https://gitlist.dev/e/20190221135805.GB20899%40sigill.intra.peff.net In-Reply-To: ``` On Mon, Feb 18, 2019 at 07:57:14PM +0100, Johannes Schindelin wrote: > > Why not use a mem_pool for this? We have that code isolated > > and re-usable now. Have mingw_getenv() copy the string into > > the pool always return the pointer from within the pool. The > > pool automatically handles allocating new blocks as necessary. > > And (if we care) we can bulk free the pool before existing. > > The problem with the mem_pool would be that I do *not* want to accumulate > tons and tons of stale copies (remember that `difftool` bug? Apparently it > calls `getenv()` in a loop that scales with the number of modified files; > I would hate to have copies for all of those). > > And all you could do to remedy this would be to blow away all of them it > the mem_pool grew too much, *including* the latest `getenv()` (which we > painfully keep valid, e.g. in the case of `git_committer_info()` which > calls `getenv()` *three* times *inside* the parameter list of the > `fmt_ident()` function call. One suggestion I made elsewhere[1] is to use strintern() to de-dup the responses, and then keep them around forever. That bounds the memory usage by the number of unique environment variables requested (which is probably finite and limited), rather than the number of getenv() calls (which as we have seen can be done in an arbitrary-sized loop). I say "probably" because it's possible that there is code which mutates the environment, reads it back, etc, in a loop. But that would be pretty out-of-character for Git, I think. -Peff [1] https://public-inbox.org/git/20190115194726.GA5818@sigill.intra.peff.net/ ```