# [BUG] [PATCH] infinite loop due to broken symlink 4 messages from 2015-03-23 to 2015-03-26. Participants: Petr Stodulka, Michael Haggerty, Junio C Hamano. Thread: https://gitlist.dev/t/38892 ## Petr Stodulka, 2015-03-23 16:04 Subject: [BUG] [PATCH] infinite loop due to broken symlink Message-ID: <5510399D.4020007@redhat.com> URL: https://gitlist.dev/e/5510399D.4020007%40redhat.com ``` Hi guys, git goes into an infinite loop due to broken symlink (minimal reproducer [0]). Affected code is in function "resolve_ref_unsafe" in file refs.c - notice 'stat_ref'. There is comment about problem with race condition, hovewer in that case it's regular broken symlink which cause infinite loop. Possible patch could be something like this: ------------------------------------------------------- diff --git a/refs.c b/refs.c index e23542b..9efe8d2 100644 --- a/refs.c +++ b/refs.c @@ -1356,6 +1356,7 @@ static struct ref_dir *get_loose_refs(struct ref_cache *refs) /* We allow "recursive" symbolic refs. Only within reason, though */ #define MAXDEPTH 5 #define MAXREFLEN (1024) +#define MAXLOOP 1024 /* * Called by resolve_gitlink_ref_recursive() after it failed to read @@ -1482,6 +1483,7 @@ const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned char buffer[256]; static char refname_buffer[256]; int bad_name = 0; + int loop_counter = 0; if (flags) *flags = 0; @@ -1546,7 +1548,8 @@ const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned if (S_ISLNK(st.st_mode)) { len = readlink(path, buffer, sizeof(buffer)-1); if (len < 0) { - if (errno == ENOENT || errno == EINVAL) + if (loop_counter++ < MAXLOOP && + (errno == ENOENT || errno == EINVAL)) /* inconsistent with lstat; retry */ goto stat_ref; else @@ -1579,7 +1582,7 @@ const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned */ fd = open(path, O_RDONLY); if (fd < 0) { - if (errno == ENOENT) + if (loop_counter++ < MAXLOOP && errno == ENOENT) /* inconsistent with lstat; retry */ goto stat_ref; else ------------------------------------------------------- If I understand well that simple check of broken symlink is not possible due to race conditions. Regards, Petr [0] https://bugzilla.redhat.com/show_bug.cgi?id=1204193 ``` ## Michael Haggerty, 2015-03-25 22:53 Subject: Re: [BUG] [PATCH] infinite loop due to broken symlink Message-ID: <55133C63.2010805@alum.mit.edu> URL: https://gitlist.dev/e/55133C63.2010805%40alum.mit.edu In-Reply-To: <5510399D.4020007@redhat.com> ``` On 03/23/2015 05:04 PM, Petr Stodulka wrote: > git goes into an infinite loop due to broken symlink (minimal reproducer > [0]). Affected code is in function > "resolve_ref_unsafe" in file refs.c - notice 'stat_ref'. There is comment > about problem with race condition, hovewer in that case it's regular broken > symlink which cause infinite loop. Thanks for the bug report. I can confirm the problem. In fact, I noticed the same problem when I was working on a refactoring in the area, but I still haven't submitted those patches. Luckily, modern Git doesn't use symlinks in the loose reference hierarchy, so most users will never encounter this problem. In fact I think it is only the open() call that can lead to the infinite loop. Meanwhile, there is another problem caused by falling through the symlink-handling code, namely that st reflects the lstat() of the symlink rather than the stat() of the file that it points to. My approach was to run stat() on the path if the symlink-handling code falls through. You can see my work in progress in my GitHub repo [1]. > Possible patch could be something > like this: > > ------------------------------------------------------- > diff --git a/refs.c b/refs.c > index e23542b..9efe8d2 100644 > --- a/refs.c > +++ b/refs.c > @@ -1356,6 +1356,7 @@ static struct ref_dir *get_loose_refs(struct > ref_cache *refs) > /* We allow "recursive" symbolic refs. Only within reason, though */ > #define MAXDEPTH 5 > #define MAXREFLEN (1024) > +#define MAXLOOP 1024 > > /* > * Called by resolve_gitlink_ref_recursive() after it failed to read > @@ -1482,6 +1483,7 @@ const char *resolve_ref_unsafe(const char > *refname, int resolve_flags, unsigned > char buffer[256]; > static char refname_buffer[256]; > int bad_name = 0; > + int loop_counter = 0; > > if (flags) > *flags = 0; > @@ -1546,7 +1548,8 @@ const char *resolve_ref_unsafe(const char > *refname, int resolve_flags, unsigned > if (S_ISLNK(st.st_mode)) { > len = readlink(path, buffer, sizeof(buffer)-1); > if (len < 0) { > - if (errno == ENOENT || errno == EINVAL) > + if (loop_counter++ < MAXLOOP && > + (errno == ENOENT || errno == EINVAL)) > /* inconsistent with lstat; > retry */ > goto stat_ref; > else > @@ -1579,7 +1582,7 @@ const char *resolve_ref_unsafe(const char > *refname, int resolve_flags, unsigned > */ > fd = open(path, O_RDONLY); > if (fd < 0) { > - if (errno == ENOENT) > + if (loop_counter++ < MAXLOOP && errno == ENOENT) > /* inconsistent with lstat; retry */ > goto stat_ref; > else > ------------------------------------------------------- > > If I understand well that simple check of broken symlink is not possible > due to race conditions. This change also prevents the infinite loop, in fact in a more failproof way that doesn't depend on errno values being interpreted correctly. I would suggest a few stylistic changes, like for example here [2]. On the other hand, this change doesn't solve the lstat()/stat() problem. Nevertheless, I wouldn't object to a fix like this being accepted in addition to the stat() fix when it's ready. It doesn't hurt to wear both a belt *and* suspenders. Michael [1] https://github.com/mhagger/git, branch "wip/refactor-resolve-ref". See especially commit d2425d8ac8cf73494b318078b92f5b1c510a68fb. [2] https://github.com/mhagger/git, branch "ref-broken-symlinks" -- Michael Haggerty mhagger@alum.mit.edu ``` ## Junio C Hamano, 2015-03-25 23:21 Subject: Re: [BUG] [PATCH] infinite loop due to broken symlink Message-ID: URL: https://gitlist.dev/e/xmqq1tkc1x49.fsf%40gitster.dls.corp.google.com In-Reply-To: <55133C63.2010805@alum.mit.edu> ``` Michael Haggerty writes: > Thanks for the bug report. I can confirm the problem. In fact, I noticed > the same problem when I was working on a refactoring in the area, but I > still haven't submitted those patches. Thanks. It is nice to know that the refs.c API refactoring is still being worked on. ``` ## Petr Stodulka, 2015-03-26 09:32 Subject: Re: [BUG] [PATCH] infinite loop due to broken symlink Message-ID: <5513D215.4070806@redhat.com> URL: https://gitlist.dev/e/5513D215.4070806%40redhat.com In-Reply-To: <55133C63.2010805@alum.mit.edu> ``` On 25.3.2015 23:53 Michael Haggerty wrote: > On 03/23/2015 05:04 PM, Petr Stodulka wrote: >> git goes into an infinite loop due to broken symlink (minimal reproducer >> [0]). Affected code is in function >> "resolve_ref_unsafe" in file refs.c - notice 'stat_ref'. There is comment >> about problem with race condition, hovewer in that case it's regular broken >> symlink which cause infinite loop. > Thanks for the bug report. I can confirm the problem. In fact, I noticed > the same problem when I was working on a refactoring in the area, but I > still haven't submitted those patches. Luckily, modern Git doesn't use > symlinks in the loose reference hierarchy, so most users will never > encounter this problem. > > In fact I think it is only the open() call that can lead to the infinite > loop. Meanwhile, there is another problem caused by falling through the > symlink-handling code, namely that st reflects the lstat() of the > symlink rather than the stat() of the file that it points to. > > My approach was to run stat() on the path if the symlink-handling code > falls through. You can see my work in progress in my GitHub repo [1]. > Yes, I thought up about similar solution, but I wasn't sure about this way because of race condition (I don't know well code of git yet). In the case of approved lstat/stat patch, I am more familiar with this solution. >> Possible patch could be something >> like this: >> >> ------------------------------------------------------- >> diff --git a/refs.c b/refs.c >> index e23542b..9efe8d2 100644 >> --- a/refs.c >> +++ b/refs.c >> @@ -1356,6 +1356,7 @@ static struct ref_dir *get_loose_refs(struct >> ref_cache *refs) >> /* We allow "recursive" symbolic refs. Only within reason, though */ >> #define MAXDEPTH 5 >> #define MAXREFLEN (1024) >> +#define MAXLOOP 1024 >> >> /* >> * Called by resolve_gitlink_ref_recursive() after it failed to read >> @@ -1482,6 +1483,7 @@ const char *resolve_ref_unsafe(const char >> *refname, int resolve_flags, unsigned >> char buffer[256]; >> static char refname_buffer[256]; >> int bad_name = 0; >> + int loop_counter = 0; >> >> if (flags) >> *flags = 0; >> @@ -1546,7 +1548,8 @@ const char *resolve_ref_unsafe(const char >> *refname, int resolve_flags, unsigned >> if (S_ISLNK(st.st_mode)) { >> len = readlink(path, buffer, sizeof(buffer)-1); >> if (len < 0) { >> - if (errno == ENOENT || errno == EINVAL) >> + if (loop_counter++ < MAXLOOP && >> + (errno == ENOENT || errno == EINVAL)) >> /* inconsistent with lstat; >> retry */ >> goto stat_ref; >> else >> @@ -1579,7 +1582,7 @@ const char *resolve_ref_unsafe(const char >> *refname, int resolve_flags, unsigned >> */ >> fd = open(path, O_RDONLY); >> if (fd < 0) { >> - if (errno == ENOENT) >> + if (loop_counter++ < MAXLOOP && errno == ENOENT) >> /* inconsistent with lstat; retry */ >> goto stat_ref; >> else >> ------------------------------------------------------- >> >> If I understand well that simple check of broken symlink is not possible >> due to race conditions. > This change also prevents the infinite loop, in fact in a more failproof > way that doesn't depend on errno values being interpreted correctly. I > would suggest a few stylistic changes, like for example here [2]. On the > other hand, this change doesn't solve the lstat()/stat() problem. > > Nevertheless, I wouldn't object to a fix like this being accepted in > addition to the stat() fix when it's ready. It doesn't hurt to wear both > a belt *and* suspenders. > > Michael > > [1] https://github.com/mhagger/git, branch "wip/refactor-resolve-ref". > See especially commit d2425d8ac8cf73494b318078b92f5b1c510a68fb. > [2] https://github.com/mhagger/git, branch "ref-broken-symlinks" > When stat/lstat is ready, probably this will not be valuable anymore, but it could be reliable 'stop' for some unexpected/unknown possibly ways in future. Petr. ```