Linus pointed out that deciphering the raw #PF error code and printing a more human readable message are two different things, and also that printing the negative cases is mostly just noise[1]. For example, the USER bit doesn't mean the fault originated in user code and stating that an oops wasn't due to a protection keys violation isn't interesting since an oops on a keys violation is a one-in-a-million scenario.
Remove the per-bit decoding of the error code and instead print: - the raw error code - why the fault occurred - the effective privilege level of the access - the type of access - whether the fault originated in user code or kernel code This provides the user with the information needed to triage 99.9% of oopses without polluting the log with useless information or conflating the error_code with the CPL. [1] https://lkml.kernel.org/r/CAHk-=whk_fsnxvmvf1t2ffcap2wrvsybabrlqcwljycvhw6...@mail.gmail.com Suggested-by: Linus Torvalds <torva...@linux-foundation.org> Cc: Andy Lutomirski <l...@kernel.org> Cc: Borislav Petkov <b...@alien8.de> Cc: Dave Hansen <dave.han...@linux.intel.com> Cc: H. Peter Anvin <h...@zytor.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Rik van Riel <r...@surriel.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Yu-cheng Yu <yu-cheng...@intel.com> Cc: linux-kernel@vger.kernel.org Cc: Ingo Molnar <mi...@kernel.org> Signed-off-by: Sean Christopherson <sean.j.christopher...@intel.com> --- v2: - Explicitly call out protection keys violations - "Slightly" reword the changelog arch/x86/mm/fault.c | 42 +++++++++++------------------------------- 1 file changed, 11 insertions(+), 31 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 2ff25ad33233..26feb8c525c1 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -603,24 +603,9 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index) name, index, addr, (desc.limit0 | (desc.limit1 << 16))); } -/* - * This helper function transforms the #PF error_code bits into - * "[PROT] [USER]" type of descriptive, almost human-readable error strings: - */ -static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt) -{ - if (error_code & mask) { - if (buf[0]) - strcat(buf, " "); - strcat(buf, txt); - } -} - static void show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long address) { - char err_txt[64]; - if (!oops_may_print()) return; @@ -648,27 +633,22 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad address < PAGE_SIZE ? "NULL pointer dereference" : "paging request", (void *)address); - err_txt[0] = 0; - - /* - * Note: length of these appended strings including the separation space and the - * zero delimiter must fit into err_txt[]. - */ - err_str_append(error_code, err_txt, X86_PF_PROT, "[PROT]" ); - err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]"); - err_str_append(error_code, err_txt, X86_PF_USER, "[USER]" ); - err_str_append(error_code, err_txt, X86_PF_RSVD, "[RSVD]" ); - err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]"); - err_str_append(error_code, err_txt, X86_PF_PK, "[PK]" ); - - pr_alert("#PF error: %s\n", error_code ? err_txt : "[normal kernel read fault]"); + pr_alert("#PF: error_code(0x%04lx) - %s\n", error_code, + !(error_code & X86_PF_PROT) ? "not-present page" : + (error_code & X86_PF_RSVD) ? "reserved bit violation" : + (error_code & X86_PF_PK) ? "protection keys violation" : + "permissions violation"); + pr_alert("#PF: %s-privileged %s from %s code\n", + (error_code & X86_PF_USER) ? "user" : "supervisor", + (error_code & X86_PF_INSTR) ? "instruction fetch" : + (error_code & X86_PF_WRITE) ? "write access" : + "read access", + user_mode(regs) ? "user" : "kernel"); if (!(error_code & X86_PF_USER) && user_mode(regs)) { struct desc_ptr idt, gdt; u16 ldtr, tr; - pr_alert("This was a system access from user code\n"); - /* * This can happen for quite a few reasons. The more obvious * ones are faults accessing the GDT, or LDT. Perhaps -- 2.19.2
