fix: starter: overwrite glibc's internal tid cache on clone() #2837
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Adapted from: opencontainers/runc#4247 Execution of a container using a PID namespace can fail on certain versions of glibc when Singularity is built with Go 1.22. This is due to Go 1.22 performing calls using pthread_self which, from glibc 2.25, is not updated for the current TID on clone. Fixes sylabs#2677 ----- Original runc explanation: Since glibc 2.25, the thread-local cache of the current TID is no longer updated in the child when calling clone(2). This results in very unfortunate behaviour when Go does pthread calls using pthread_self(), which has the wrong TID stored. The "simple" solution is to forcefully overwrite this cached value. Unfortunately (and unsurprisingly), the layout of "struct pthread" is strictly private and can change without warning. Luckily, glibc (currently) uses CLONE_CHILD_CLEARTID for all forks (with the child_tid set to the cached &PTHREAD_SELF->tid), meaning that as long as runc is using glibc, when "runc init" is spawned the child process will have a pointer directly to the cached value we want to change. With CONFIG_CHECKPOINT_RESTORE=y kernels on Linux 3.5 and later, we can simply use prctl(PR_GET_TID_ADDRESS). For older kernels we need to memory scan the TLS structure (pthread_self() returns a pointer to the start of the structure so we can "just" scan it for a field containing the current TID and assume that it is the correct field). Obviously this is all very horrific, and if you are reading this in the future, it almost certainly has caused some horrific bug that I did not forsee. Sorry about that. As far as I can tell, there is no other workable solution that doesn't also depend on the CLONE_CHILD_CLEARTID behaviour of glibc in some way. We cannot "just" do a re-exec after clone(2) for security reasons. Fixes opencontainers/runc#4233 Signed-off-by: Aleksa Sarai cyphar@cyphar.com
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Adapted from: opencontainers/runc#4247
Execution of a container using a PID namespace can fail on certain versions of glibc when Singularity is built with Go 1.22.
This is due to Go 1.22 performing calls using pthread_self which, from glibc 2.25, is not updated for the current TID on clone.
Fixes #2677
Original runc explanation:
Since glibc 2.25, the thread-local cache of the current TID is no longer updated in the child when calling clone(2). This results in very unfortunate behaviour when Go does pthread calls using pthread_self(), which has the wrong TID stored.
The "simple" solution is to forcefully overwrite this cached value. Unfortunately (and unsurprisingly), the layout of "struct pthread" is strictly private and can change without warning.
Luckily, glibc (currently) uses CLONE_CHILD_CLEARTID for all forks (with the child_tid set to the cached &PTHREAD_SELF->tid), meaning that as long as runc is using glibc, when "runc init" is spawned the child process will have a pointer directly to the cached value we want to change. With CONFIG_CHECKPOINT_RESTORE=y kernels on Linux 3.5 and later, we can simply use prctl(PR_GET_TID_ADDRESS). For older kernels we need to memory scan the TLS structure (pthread_self() returns a pointer to the start of the structure so we can "just" scan it for a field containing the current TID and assume that it is the correct field).
Obviously this is all very horrific, and if you are reading this in the future, it almost certainly has caused some horrific bug that I did not forsee. Sorry about that. As far as I can tell, there is no other workable solution that doesn't also depend on the CLONE_CHILD_CLEARTID behaviour of glibc in some way. We cannot "just" do a re-exec after clone(2) for security reasons.
Fixes opencontainers/runc#4233 Signed-off-by: Aleksa Sarai cyphar@cyphar.com
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