From 2f5f0eb016411170ecaabe019e062977ee6ec2d2 Mon Sep 17 00:00:00 2001 From: Brian Gerst Date: Fri, 21 Jul 2023 12:10:12 -0400 Subject: [PATCH] x86/entry/64: Remove obsolete comment on tracing vs. SYSRET [ Upstream commit eb43c9b1517b48e2ff0d3a584aca197338987d7b ] This comment comes from a time when the kernel attempted to use SYSRET on all returns to userspace, including interrupts and exceptions. Ever since commit fffbb5dc ("Move opportunistic sysret code to syscall code path"), SYSRET is only used for returning from system calls. The specific tracing issue listed in this comment is not possible anymore. Signed-off-by: Brian Gerst Signed-off-by: Ingo Molnar Cc: Andy Lutomirski Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Josh Poimboeuf Link: https://lore.kernel.org/r/20230721161018.50214-2-brgerst@gmail.com Stable-dep-of: ac8b270b61d4 ("x86/bhi: Avoid warning in #DB handler due to BHI mitigation") Signed-off-by: Sasha Levin --- arch/x86/entry/entry_64.S | 19 +++---------------- 1 file changed, 3 insertions(+), 16 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 1edb8e1b9e01..2192b6c33ea0 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -167,22 +167,9 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL) jne swapgs_restore_regs_and_return_to_usermode /* - * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot - * restore RF properly. If the slowpath sets it for whatever reason, we - * need to restore it correctly. - * - * SYSRET can restore TF, but unlike IRET, restoring TF results in a - * trap from userspace immediately after SYSRET. This would cause an - * infinite loop whenever #DB happens with register state that satisfies - * the opportunistic SYSRET conditions. For example, single-stepping - * this user code: - * - * movq $stuck_here, %rcx - * pushfq - * popq %r11 - * stuck_here: - * - * would never get past 'stuck_here'. + * SYSRET cannot restore RF. It can restore TF, but unlike IRET, + * restoring TF results in a trap from userspace immediately after + * SYSRET. */ testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11 jnz swapgs_restore_regs_and_return_to_usermode