commit 088c48490555cd05a2250ea88234ee373e027dc1 tree ab5c84b88eb2722dd80213ab855cc35a52c181cd parent bb0502898c0805dfffbc3577fce8881c60ae03cc author Li Wang 2026-01-21 17:01:59 +0800 committer Li Wang 2026-01-22 08:39:17 +0800 lapi/tls: remove the TLS support from i386 Using a LTP naked clone() to verify that CLONE_SETTLS is unreliable when running 32-bit on x86_64, since TLS requires two steps: writing the descriptor and switching the selector. But CLONE_SETTLS on i386 only overrides the former: kernel_clone() copy_process() copy_thread() set_new_tls() do_set_thread_area() In copy_thread(), the child's register frame is copied from the parent *childregs = *current_pt_regs(); and on the 32-bit side it also does savesegment(gs, p->thread.gs); saving the current %gs into thread_struct. Together, this means that unless something explicitly overwrites it later, the child's initial %gs selector is inherited from the parent. https://elixir.bootlin.com/linux/v6.18/source/arch/x86/kernel/process.c#L243 Then, in do_set_thread_area(), the kernel updates the TLS descriptor set_tls_desc(p, idx, &info, 1); However, when (p != current), the x86_32 path does not update or refresh any segment selector. So it updates the descriptor but does not switch the child's %gs selector to the new modified_sel. https://elixir.bootlin.com/linux/v6.18/source/arch/x86/kernel/tls.c#L150 Therefore, on i386, relying on CLONE_SETTLS alone can leave the child executing with the parent's %gs selector, so TLS accesses still resolve to the old TLS base. =============== The behavior above explains why clone10 fails even if we update the TLS descriptor base (either hard-coding or via set_thread_area()). Example (x86_64 kernel running a 32-bit ELF): # uname -rm 6.19.0-rc2.liwang x86_64 # readelf -h clone10 |grep Class Class: ELF32 # ./clone10 ... clone10.c:48: TINFO: Child (PID: 5262, TID: 5263): TLS value set to: 101 clone10.c:72: TFAIL: Parent (PID: 5262, TID: 5262): TLS value mismatch: got 101, expected 100 Reported-by: Wei Gao Signed-off-by: Li Wang Reviewed-by: Wei Gao Acked-by: Cyril Hrubis :100644 100644 a067872e0f3a93d4e7bb7563884d1d0032d3e0ce f7e2f483fe9e623e669d3f0e9f09d9218b42f216 M include/lapi/tls.h