nss_cache: implement initgroups_dyn - #113
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kmjohansen wants to merge 1 commit into
Open
kmjohansen wants to merge 1 commit into
kmjohansen wants to merge 1 commit into
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glibc's libnss has a callback for initgroups(3) which it uses preferentially to getgrent_r. The getgrent_r fallback is used if initgroups_dyn is not implemented, but glibc does not have any serialization around the callback. This means that if multiple threads call initgroups simultaneously and the nss backend does not implment initgroups_dyn, then it's possible the supplemental group lists will become truncated as multiple getgrent_r calls race with one another. Fix this in libnss_cache by implementing an initgroups_dyn callback. This callback is written in the style of nss-files but with the libnss-cache idioms. It holds the cache lock across the getgrent_r iteration to ensure no supplemental group calls are truncated. This was tested by reproducing the truncation race with an fresh nscd that is bombarded with id -G calls. Without this fix present, it's possible to observe truncated results. However, with initgroups_dyn users always get their correct supplemental groups.
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glibc's libnss has a callback for initgroups(3) which it uses preferentially to getgrent_r. The getgrent_r fallback is used if initgroups_dyn is not implemented, but glibc does not have any serialization around the callback. This means that if multiple threads call initgroups simultaneously and the nss backend does not implment initgroups_dyn, then it's possible the supplemental group lists will become truncated as multiple getgrent_r calls race with one another.
Fix this in libnss_cache by implementing an initgroups_dyn callback. This callback is written in the style of nss-files but with the libnss-cache idioms. It holds the cache lock across the getgrent_r iteration to ensure no supplemental group calls are truncated.
This was tested by reproducing the truncation race with an fresh nscd that is bombarded with id -G calls. Without this fix present, it's possible to observe truncated results. However, with initgroups_dyn users always get their correct supplemental groups.