[BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[David Michael]

It is possible for this pointer of the GIT_DIR environment variable to
survive unduplicated until further getenv calls are made.  The standards
allow for subsequent calls of getenv to overwrite the string located at
its returned pointer, and this can result in broken git operations on
certain platforms.

Signed-off-by: David Michael <fedora.dm0@gmail.com>
---

I have encountered an issue with consecutive calls to getenv
overwriting earlier values.  Most notably, it prevents a plain "git
clone" from working.

Long story short: This value of GIT_DIR gets passed around setup.c
until it reaches check_repository_format_gently.  This function calls
git_config_early, which eventually runs getenv("HOME").  When it
returns back to check_repository_format_gently, the gitdir variable
contains my home directory path.  The end result is that I wind up
with ~/objects/ etc. and a failed repository clone.  (Simply adding a
bare getenv("GIT_DIR") afterwards to reset the pointer also corrects
the problem.)

Since other platforms are apparently working, yet this getenv behavior
is supported by the standards, I am left wondering if this could be a
symptom of something else being broken on my platform (z/OS).  Can
anyone more familiar with this part of git identify any condition that
obviously should not be occurring?

Thanks.

 setup.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/setup.c b/setup.c
index f108c4b..64fb160 100644
--- a/setup.c
+++ b/setup.c
@@ -675,8 +675,12 @@ static const char
*setup_git_directory_gently_1(int *nongit_ok)
      * validation.
      */
     gitdirenv = getenv(GIT_DIR_ENVIRONMENT);
-    if (gitdirenv)
-        return setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
+    if (gitdirenv) {
+        gitdirenv = xstrdup(gitdirenv);
+        ret = setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
+        free(gitdirenv);
+        return ret;
+    }

     if (env_ceiling_dirs) {
         string_list_split(&ceiling_dirs, env_ceiling_dirs, PATH_SEP, -1);
--
1.7.11.7

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Junio C Hamano]

David Michael <fedora.dm0@gmail.com> writes:

> I have encountered an issue with consecutive calls to getenv
> overwriting earlier values.  Most notably, it prevents a plain "git
> clone" from working.
>
> Long story short: This value of GIT_DIR gets passed around setup.c
> until it reaches check_repository_format_gently.  This function calls
> git_config_early, which eventually runs getenv("HOME").  When it
> returns back to check_repository_format_gently, the gitdir variable
> contains my home directory path.  The end result is that I wind up
> with ~/objects/ etc. and a failed repository clone.  (Simply adding a
> bare getenv("GIT_DIR") afterwards to reset the pointer also corrects
> the problem.)
>
> Since other platforms are apparently working, yet this getenv behavior
> is supported by the standards, I am left wondering if this could be a
> symptom of something else being broken on my platform (z/OS).

The execve(2) function

       int execve(const char *filename, char *const argv[],
                  char *const envp[]);

takes a NULL terminated array of NUL terminated strings of form
"VAR=VAL" in envp[], and this is kept in:

	extern char **environ;

of the new image that runs.

The most naive and straight-forward way to implement getenv(3) is to
iterate over this environ[] array to look for an element that begins
with "GIT_DIR=", and return the pointer pointing at the location one
byte past that equal sign.  So even if the standard allowed the
returned value to be volatile across calls to getenv(3), it will
take *more* work for implementations if they want to break our use
pattern.  They have to deliberately return a string that they will
overwrite in subsequent calls to getenv(3).

Also the natural way to implement putenv(3) and setenv(3) is to
replace the pointer in the environ[] array (not overwrite the
existing string in environ[] that holds the "VAR=VAL" string that
represents the current value, which might be shorter than the new
value of the enviornment variable), hence even calling these
functions is unlikely to invalidate the result you previously
received from getenv(3).

I am not at all surprised that nobody from other platforms has seen
this breakage.

In fact,

    http://pubs.opengroup.org/onlinepubs/9699919799/functions/getenv.html

says that only setenv(), unsetenv() and putenv() may invalidate
previous return values.  Note that getenv() is not listed as a
function that is allowed to break return values from a previous call
to getenv().

So in short, your platform's getenv(3) emulation may be broken.  We
have other calls to getenv() where we rely on the result of it being
stable, and you might discover these places also break.

Having said that, we do have codepaths to update a handful of
environment variables ourselves (GIT_DIR is among them), so I think
your patch is a good safety measure in general.

>  setup.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/setup.c b/setup.c
> index f108c4b..64fb160 100644
> --- a/setup.c
> +++ b/setup.c
> @@ -675,8 +675,12 @@ static const char
> *setup_git_directory_gently_1(int *nongit_ok)
>       * validation.
>       */
>      gitdirenv = getenv(GIT_DIR_ENVIRONMENT);
> -    if (gitdirenv)
> -        return setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
> +    if (gitdirenv) {
> +        gitdirenv = xstrdup(gitdirenv);
> +        ret = setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
> +        free(gitdirenv);
> +        return ret;
> +    }
>
>      if (env_ceiling_dirs) {
>          string_list_split(&ceiling_dirs, env_ceiling_dirs, PATH_SEP, -1);
> --
> 1.7.11.7

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[David Michael]

Hi,

On Fri, Jan 4, 2013 at 8:17 PM, Junio C Hamano <gitster@pobox.com> wrote:
> In fact,
>
>     http://pubs.opengroup.org/onlinepubs/9699919799/functions/getenv.html
>
> says that only setenv(), unsetenv() and putenv() may invalidate
> previous return values.  Note that getenv() is not listed as a
> function that is allowed to break return values from a previous call
> to getenv().

Before I sent the e-mail, I checked that very page to be sure I wasn't
entirely insane.  Specifically, the second paragraph begins with:

> The string pointed to may be overwritten by a subsequent call to getenv(), [...]

I read that line as confirmation that this is indeed acceptably
standard behavior.  Even the getenv man page on my Fedora workstation
says:

> The string pointed to by the return value of getenv() may be statically allocated, and can be modified by a subsequent call to getenv(), putenv(3), setenv(3), or unsetenv(3).

Am I misinterpreting these statements?

Thanks.

David

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Junio C Hamano]

Junio C Hamano <gitster@pobox.com> writes:

> ...  So even if the standard allowed the
> returned value to be volatile across calls to getenv(3),...
> ...
> In fact,
>
>     http://pubs.opengroup.org/onlinepubs/9699919799/functions/getenv.html
>
> says that only ...

Apparently I wasn't even reading what I was quoting carefully
enough.  The above does include getenv() as one of the functions
that are allowed to invalidate earlier return values.

Sorry about that.  I'll go back to bed (I am a bit under the weather
and OOO today).  The conclusion in my original message is still
valid.

> Having said that, we do have codepaths to update a handful of
> environment variables ourselves (GIT_DIR is among them), so I think
> your patch is a good safety measure in general.

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Duy Nguyen]

On Sat, Jan 5, 2013 at 7:35 AM, David Michael <fedora.dm0@gmail.com> wrote:
> -    if (gitdirenv)
> -        return setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
> +    if (gitdirenv) {
> +        gitdirenv = xstrdup(gitdirenv);
> +        ret = setup_explicit_git_dir(gitdirenv, cwd, len, nongit_ok);
> +        free(gitdirenv);
> +        return ret;
> +    }

Maybe we could all this into a wrapper? If getenv() here has a
problem, many other places may have the same problem too. This
simplifies the change. But one has to check that getenv() must not be
used in threaded code.

char *git_getenv(const char *env)
{
   static int bufno;
   static char *buf[4];
   bufno = (bufno + 1) % 4;
   free(buf[bufno]);
   buf[bufno] = xstrdup(getenv(env));
   return buf[bufno];
}
#define getenv(x) git_getenv(x)

-- 
Duy

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Junio C Hamano]

Duy Nguyen <pclouds@gmail.com> writes:

> Maybe we could all this into a wrapper? If getenv() here has a
> problem, many other places may have the same problem too. This
> simplifies the change. But one has to check that getenv() must not be
> used in threaded code.

That needs to be done regardless, if we care; POSIX explicitly says
getenv() need not be thread-safe.

I personally do not think a wrapper with limited slots is a healthy
direction to go.  Most places we use getenv() do not let the return
value live across their scope, and those that do should explicitly
copy the value away.  It's between validating that there is _no_ *env()
calls in the codepath between a getenv() call and the use of its
return value, and validating that there is at most 4 such calls there.
The former is much easier to verify and maintain, I think.

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Duy Nguyen]

On Sat, Jan 5, 2013 at 11:38 AM, Junio C Hamano <gitster@pobox.com> wrote:
> I personally do not think a wrapper with limited slots is a healthy
> direction to go.  Most places we use getenv() do not let the return
> value live across their scope, and those that do should explicitly
> copy the value away.  It's between validating that there is _no_ *env()
> calls in the codepath between a getenv() call and the use of its
> return value, and validating that there is at most 4 such calls there.
> The former is much easier to verify and maintain, I think.

I did not look carefully and was scared of 143 getenv calls. But with
about 4 calls, yes it's best to do without the wrapper.
-- 
Duy

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Junio C Hamano]

Duy Nguyen <pclouds@gmail.com> writes:

> On Sat, Jan 5, 2013 at 11:38 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> I personally do not think a wrapper with limited slots is a healthy
>> direction to go.  Most places we use getenv() do not let the return
>> value live across their scope, and those that do should explicitly
>> copy the value away.  It's between validating that there is _no_ *env()
>> calls in the codepath between a getenv() call and the use of its
>> return value, and validating that there is at most 4 such calls there.
>> The former is much easier to verify and maintain, I think.
>
> I did not look carefully and was scared of 143 getenv calls. But with
> about 4 calls, yes it's best to do without the wrapper.

Just to make sure you did not misunderstand, the 4 (four) in my
message is not about "4 calls among 143 are unsafe".

It was referring to the number of rotating slots your patch defined,
which means

	val = getenv("FOO");
        ... random other code ...
        use(val)

is safe only if random other code makes less than 4 getenv() calls.

I didn't verify all of the call sites. It needs to be done with or
without your wrapper patch. Without your wrapper, the validation
needs to make sure "random other code" above does not make any getenv()
call.  With your wrapper, the validation needs to make sure "random
other code" above does not make more than 4 such calls.  My point
was that the effort needed for both are about the same, so your
wrapper does not buy us much.

[PATCH] Add getenv.so for catching invalid getenv() use via LD_PRELOAD

[Nguyễn Thái Ngọc Duy]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Perhaps this will help the getenv bug hunting (I assume we do the
 hunting on Linux platform only). So far it catches this and is stuck
 at getenv in git_pager().

  diff --git a/exec_cmd.c b/exec_cmd.c
  index 125fa6f..d8be5ce 100644
  --- a/exec_cmd.c
  +++ b/exec_cmd.c
  @@ -97,7 +97,7 @@ static void add_path(struct strbuf *out, const char *path)
   
   void setup_path(void)
   {
  -       const char *old_path = getenv("PATH");
  +       char *old_path = xstrdup(getenv("PATH"));
          struct strbuf new_path = STRBUF_INIT;
   
          add_path(&new_path, git_exec_path());
  @@ -110,6 +110,7 @@ void setup_path(void)
   
          setenv("PATH", new_path.buf, 1);
   
  +       free(old_path);
          strbuf_release(&new_path);
   }

 contrib/getenv/Makefile |  2 ++
 contrib/getenv/getenv.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)
 create mode 100644 contrib/getenv/Makefile
 create mode 100644 contrib/getenv/getenv.c

diff --git a/contrib/getenv/Makefile b/contrib/getenv/Makefile
new file mode 100644
index 0000000..4881b85
--- /dev/null
+++ b/contrib/getenv/Makefile
@@ -0,0 +1,2 @@
+getenv.so: getenv.c
+	$(CC) -g -shared -fPIC -ldl -o $@ $<
diff --git a/contrib/getenv/getenv.c b/contrib/getenv/getenv.c
new file mode 100644
index 0000000..e351e10
--- /dev/null
+++ b/contrib/getenv/getenv.c
@@ -0,0 +1,67 @@
+#include <gnu/lib-names.h>
+#include <sys/mman.h>
+#include <dlfcn.h>
+#include <execinfo.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+
+/* Global symbols for easy access from gdb */
+static char *getenv_current;
+static char *getenv_prev;
+
+/*
+ * Intercept standard getenv() via LD_PRELOAD. The return value is
+ * made inaccessible by the next getenv() call. This helps catch
+ * places that ignore the statement "The string pointed to may be
+ * overwritten by a subsequent call to getenv()" [1].
+ *
+ * The backtrace is appended after the env string, which may be
+ * helpful to identify where this getenv() is called in a core dump.
+ *
+ * [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/getenv.html
+ */
+char *getenv(const char *name)
+{
+	static char *(*libc_getenv)(const char*);
+	char *value;
+
+	if (!libc_getenv) {
+		void *libc = dlopen(LIBC_SO, RTLD_LAZY);
+		libc_getenv = dlsym(libc, "getenv");
+	}
+	if (getenv_current) {
+		mprotect(getenv_current, strlen(getenv_current) + 1, PROT_NONE);
+		getenv_prev = getenv_current;
+		getenv_current = NULL;
+	}
+
+	value = libc_getenv(name);
+	if (value) {
+		int len = strlen(value) + 1;
+		int backtrace_len = 0;
+		void *buffer[100];
+		char **symbols;
+		int i, n;
+
+		n = backtrace(buffer, 100);
+		symbols = backtrace_symbols(buffer, n);
+		if (symbols) {
+			for (i = 0;i < n; i++)
+				backtrace_len += strlen(symbols[i]) + 1; /* \n */
+			backtrace_len++; /* NULL */
+		}
+
+		getenv_current = mmap(NULL, len + backtrace_len, PROT_READ | PROT_WRITE,
+				   MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
+		memcpy(getenv_current, value, len);
+		value = getenv_current;
+
+		if (symbols) {
+			char *p = getenv_current + len;
+			for (i = 0; i < n; i++)
+				p += sprintf(p, "%s\n", symbols[i]);
+		}
+	}
+	return value;
+}
-- 
1.8.0.rc2.23.g1fb49df

Re: [PATCH] Add getenv.so for catching invalid getenv() use via LD_PRELOAD

[Matt Kraai]

On Sat, Jan 05, 2013 at 03:55:46PM +0700, Nguyễn Thái Ngọc Duy wrote:
>  Perhaps this will help the getenv bug hunting (I assume we do the
>  hunting on Linux platform only). So far it catches this and is stuck
>  at getenv in git_pager().

It seems like a static analysis tool might be able to detect these
problems.  Is there a way to do so using sparse?

> +		n = backtrace(buffer, 100);
> +		symbols = backtrace_symbols(buffer, n);
> +		if (symbols) {
> +			for (i = 0;i < n; i++)

s/;i/; i/

-- 
Matt Kraai
https://ftbfs.org/kraai

Re: [PATCH] Add getenv.so for catching invalid getenv() use via LD_PRELOAD

[Duy Nguyen]

On Sat, Jan 5, 2013 at 5:39 PM, Matt Kraai <kraai@ftbfs.org> wrote:
> On Sat, Jan 05, 2013 at 03:55:46PM +0700, Nguyễn Thái Ngọc Duy wrote:
>>  Perhaps this will help the getenv bug hunting (I assume we do the
>>  hunting on Linux platform only). So far it catches this and is stuck
>>  at getenv in git_pager().
>
> It seems like a static analysis tool might be able to detect these
> problems.  Is there a way to do so using sparse?

That was my first thought. But this may involve flow analysis and I
don't think sparse is up to it. ccc-analyzer is still pretty basic.
And between static analysis and runtime check, I prefer the latter as
it's more reliable as long as you have a good coverage test.

>
>> +             n = backtrace(buffer, 100);
>> +             symbols = backtrace_symbols(buffer, n);
>> +             if (symbols) {
>> +                     for (i = 0;i < n; i++)
>
> s/;i/; i/

Thanks. I will fix it later if people actually want this.
-- 
Duy

Re: [PATCH] Add getenv.so for catching invalid getenv() use via LD_PRELOAD

[Jonathan Nieder]

Nguyễn Thái Ngọc Duy wrote:

> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  Perhaps this will help the getenv bug hunting

Even if no one decides to do the getenv hunting (I haven't decided yet
whether it's worth the trouble, though patches like the setup_path()
one that make string lifetimes clearer are valuable anyway), this
looks useful as a debugging tool when people on strange platforms
report problems.  And unlike the trick I sent a while ago, it doesn't
involve recompiling git.  So for what it's worth,

Acked-by: Jonathan Nieder <jrnieder@gmail.com>

Thanks.

Re: [PATCH] Add getenv.so for catching invalid getenv() use via LD_PRELOAD

[David Michael]

Hi,

On Sat, Jan 5, 2013 at 3:55 AM, Nguyễn Thái Ngọc Duy <pclouds@gmail.com> wrote:
>  Perhaps this will help the getenv bug hunting (I assume we do the
>  hunting on Linux platform only). So far it catches this and is stuck
>  at getenv in git_pager().

For the record: I have been testing a macro pointing getenv at an
alternate implementation for the purpose of my port.  This alternate
function will return a unique pointer for each variable, as opposed to
using the same static buffer.  IBM considers this function a
proprietary z/OS extension (whereas the other is labelled as
conforming to half a dozen standards), but it seems to be more in-line
with the behavior git expects.

So I agree this patch is useful for reaching strict conformance to the
published specs, but I think we can go back to assuming no known
platforms are broken and unusable because of this.

Thanks.

David

Re: [BUG/PATCH] setup: Copy an environment variable to avoid overwrites

[Erik Faye-Lund]

On Sat, Jan 5, 2013 at 1:35 AM, David Michael <fedora.dm0@gmail.com> wrote:
> It is possible for this pointer of the GIT_DIR environment variable to
> survive unduplicated until further getenv calls are made.  The standards
> allow for subsequent calls of getenv to overwrite the string located at
> its returned pointer, and this can result in broken git operations on
> certain platforms.
>
> Signed-off-by: David Michael <fedora.dm0@gmail.com>
> ---
>
> I have encountered an issue with consecutive calls to getenv
> overwriting earlier values.  Most notably, it prevents a plain "git
> clone" from working.
>
> Long story short: This value of GIT_DIR gets passed around setup.c
> until it reaches check_repository_format_gently.  This function calls
> git_config_early, which eventually runs getenv("HOME").  When it
> returns back to check_repository_format_gently, the gitdir variable
> contains my home directory path.  The end result is that I wind up
> with ~/objects/ etc. and a failed repository clone.  (Simply adding a
> bare getenv("GIT_DIR") afterwards to reset the pointer also corrects
> the problem.)
>
> Since other platforms are apparently working, yet this getenv behavior
> is supported by the standards, I am left wondering if this could be a
> symptom of something else being broken on my platform (z/OS).  Can
> anyone more familiar with this part of git identify any condition that
> obviously should not be occurring?
>
> Thanks.

I have some patches of a similar nature here:

https://github.com/kusma/git/commits/work/getenv-safety

These were written for an earlier version of the UTF-8 patches for Git
for Windows, where we were looking into allowing getenv to use a
static buffer to convert the environment variables from UTF-16 (which
is what Windows maintains) to UTF-8. We ended converting the
environment on start-up instead, so these weren't needed for us. But
perhaps they can be of use to someone else?