On Thu, Nov 22, 2018 at 09:41:19AM +0100, Ingo Molnar wrote: > > * Andy Lutomirski <[email protected]> wrote: > > > One of Linus' favorite hobbies seems to be looking at OOPSes and > > decoding the error code in his head. This is not one of my favorite > > hobbies :) > > > > Teach the page fault OOPS hander to decode the error code. If it's > > a !USER fault from user mode, print an explicit note to that effect > > and print out the addresses of various tables that might cause such > > an error. > > > > With this patch applied, if I intentionally point the LDT at 0x0 and > > run the x86 selftests, I get: > > > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 > > HW error: normal kernel read fault > > This was a system access from user code > > IDT: 0xfffffe0000000000 (limit=0xfff) GDT: 0xfffffe0000001000 (limit=0x7f) > > LDTR: 0x50 -- base=0x0 limit=0xfff7 > > TR: 0x40 -- base=0xfffffe0000003000 limit=0x206f > > PGD 800000000456e067 P4D 800000000456e067 PUD 4623067 PMD 0 > > SMP PTI > > CPU: 0 PID: 153 Comm: ldt_gdt_64 Not tainted 4.19.0+ #1317 > > Hardware name: ... > > RIP: 0033:0x401454 > > I've applied your series, with one small edit, the following message: > > > HW error: normal kernel read fault > > will IMHO confuse the heck out of users, thinking that their hardware is > broken... > > Yes, the message is accurate, in MM pagefault language it's indeed the HW > error code, but it's a language very few people speak. > > So I edited it over to say '#PF error code'. I also applied a few other > minor cleanups - see the changelog below.
I responded to the original thread a hair too late... What about something like this instead of manually handling the case where error_code==0 so that we get e.g. "[KERNEL] [READ]" instead of "normal kernel read fault"? Getting "[PROT] [KERNEL] [READ]" seems useful. IMO "[normal kernel read fault]" followed by "This was a system access from user code" is still confusing. --- 8b29ee4351d5c625aa9ca2765f8da5e Mon Sep 17 00:00:00 2001 From: Sean Christopherson <[email protected]> Date: Tue, 27 Nov 2018 07:09:57 -0800 Subject: [PATCH] x86/fault: Print "KERNEL" and "READ" for #PF error codes ...and explicitly state that it's a "code" that's being printed. Cc: Andy Lutomirski <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Dave Hansen <[email protected]> Cc: H. Peter Anvin <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Yu-cheng Yu <[email protected]> Cc: [email protected] Cc: Ingo Molnar <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> --- arch/x86/mm/fault.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 2ff25ad33233..510e263c256b 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -660,8 +660,10 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad 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]"); + err_str_append(~error_code, err_txt, X86_PF_USER, "[KERNEL]"); + err_str_append(~error_code, err_txt, X86_PF_WRITE | X86_PF_INSTR, + "[READ]"); + pr_alert("#PF error code: %s\n", err_txt); if (!(error_code & X86_PF_USER) && user_mode(regs)) { struct desc_ptr idt, gdt; -- 2.19.2 > > Let me know if you have any objections. > > Thanks, > > Ingo > > ===============> > From a2aa52ab16efbee40ad118ebac4a5e438f5b43ee Mon Sep 17 00:00:00 2001 > From: Ingo Molnar <[email protected]> > Date: Thu, 22 Nov 2018 09:34:03 +0100 > Subject: [PATCH] x86/fault: Clean up the page fault oops decoder a bit > > - Make the oops messages a bit less scary (don't mention 'HW errors') > > - Turn 'PROT USER' (which is visually easily confused with PROT_USER) > into individual bit descriptors: "[PROT] [USER]". > This also makes "[normal kernel read fault]" more apparent. > > - De-abbreviate variables to make the code easier to read > > - Use vertical alignment where appropriate. > > - Add comment about string size limits and the helper function. > > - Remove unnecessary line breaks. > > Cc: Andy Lutomirski <[email protected]> > Cc: Borislav Petkov <[email protected]> > Cc: Dave Hansen <[email protected]> > Cc: H. Peter Anvin <[email protected]> > Cc: Linus Torvalds <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Rik van Riel <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Yu-cheng Yu <[email protected]> > Cc: [email protected] > Signed-off-by: Ingo Molnar <[email protected]> > --- > arch/x86/mm/fault.c | 38 +++++++++++++++++++++++--------------- > 1 file changed, 23 insertions(+), 15 deletions(-) > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index f5efbdba2b6d..2ff25ad33233 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -603,10 +603,13 @@ static void show_ldttss(const struct desc_ptr *gdt, > const char *name, u16 index) > name, index, addr, (desc.limit0 | (desc.limit1 << 16))); > } > > -static void errstr(unsigned long ec, char *buf, unsigned long mask, > - const char *txt) > +/* > + * 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 (ec & mask) { > + if (error_code & mask) { > if (buf[0]) > strcat(buf, " "); > strcat(buf, txt); > @@ -614,10 +617,9 @@ static void errstr(unsigned long ec, char *buf, unsigned > long mask, > } > > static void > -show_fault_oops(struct pt_regs *regs, unsigned long error_code, > - unsigned long address) > +show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned > long address) > { > - char errtxt[64]; > + char err_txt[64]; > > if (!oops_may_print()) > return; > @@ -646,15 +648,21 @@ show_fault_oops(struct pt_regs *regs, unsigned long > error_code, > address < PAGE_SIZE ? "NULL pointer dereference" : "paging > request", > (void *)address); > > - errtxt[0] = 0; > - errstr(error_code, errtxt, X86_PF_PROT, "PROT"); > - errstr(error_code, errtxt, X86_PF_WRITE, "WRITE"); > - errstr(error_code, errtxt, X86_PF_USER, "USER"); > - errstr(error_code, errtxt, X86_PF_RSVD, "RSVD"); > - errstr(error_code, errtxt, X86_PF_INSTR, "INSTR"); > - errstr(error_code, errtxt, X86_PF_PK, "PK"); > - pr_alert("HW error: %s\n", error_code ? errtxt : > - "normal kernel read fault"); > + 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]"); > + > if (!(error_code & X86_PF_USER) && user_mode(regs)) { > struct desc_ptr idt, gdt; > u16 ldtr, tr;
