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    Subject[PATCH] x86/mm/fault: Streamline the fault error_code decoder some more

    * Sean Christopherson <sean.j.christopherson@intel.com> wrote:

    > ...instead of manually handling the case where error_code=0, e.g. to
    > display "[SUPERVISOR] [READ]" instead of "normal kernel read fault".
    >
    > This makes the zero case consistent with all other messages and also
    > provides additional information for other error code combinations,
    > e.g. error_code==1 will display "[PROT] [SUPERVISOR] [READ]" instead
    > of simply "[PROT]".
    >
    > Print unique names for the negative cases as opposed to e.g. "[!USER]"
    > to avoid mixups due to users missing a single "!" character, and to be
    > more concise for the !INSTR && !WRITE case.
    >
    > Print "SUPERVISOR" in favor of "KERNEL" to reduce the likelihood that
    > the message is misinterpreted as a generic kernel/software error and
    > to be consistent with the SDM's nomenclature.
    >
    > An alternative to passing a negated error code to err_str_append() would
    > be to expand err_str_append() to take a second string for the negative
    > test, but that approach complicates handling the "[READ]" case, which
    > looks for !INSTR && !WRITE, e.g. it would require an extra call to
    > err_str_append() and logic in err_str_append() to allow null messages
    > for both the positive and negative tests. Printing "[INSTR] [READ]"
    > wouldn't be the end of the world, but a little bit of trickery in the
    > kernel is a relatively small price to pay in exchange for the ability
    > to unequivocally know the access type by reading a single word.
    >
    > Now that all components of the message use the [<code>] format,
    > explicitly state that it's the error *code* that's being printed and
    > group the err_str_append() calls by type so that the resulting print
    > messages are consistent, e.g. the deciphered codes will always be:
    >
    > [PROT] [USER|SUPERVISOR] [WRITE|INSTR|READ] [RSDV] [PK]
    >
    > Cc: Andy Lutomirski <luto@kernel.org>
    > Cc: Borislav Petkov <bp@alien8.de>
    > Cc: Dave Hansen <dave.hansen@linux.intel.com>
    > Cc: H. Peter Anvin <hpa@zytor.com>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Rik van Riel <riel@surriel.com>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
    > Cc: linux-kernel@vger.kernel.org
    > Cc: Ingo Molnar <mingo@kernel.org>
    > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    > ---
    > arch/x86/mm/fault.c | 11 +++++++----
    > 1 file changed, 7 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
    > index 2ff25ad33233..0b4ce5d2b461 100644
    > --- a/arch/x86/mm/fault.c
    > +++ b/arch/x86/mm/fault.c
    > @@ -609,7 +609,7 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
    > */
    > static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt)
    > {
    > - if (error_code & mask) {
    > + if ((error_code & mask) == mask) {
    > if (buf[0])
    > strcat(buf, " ");
    > strcat(buf, txt);
    > @@ -655,13 +655,16 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
    > * 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_USER, "[SUPERVISOR]");
    > + err_str_append(error_code, err_txt, X86_PF_WRITE, "[WRITE]");
    > err_str_append(error_code, err_txt, X86_PF_INSTR, "[INSTR]");
    > + err_str_append(~error_code, err_txt, X86_PF_WRITE | X86_PF_INSTR,
    > + "[READ]");
    > + err_str_append(error_code, err_txt, X86_PF_RSVD, "[RSVD]" );
    > 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: %s\n", err_txt);
    >
    > if (!(error_code & X86_PF_USER) && user_mode(regs)) {
    > struct desc_ptr idt, gdt;

    Yeah, so I don't like the overly long 'SUPERVISOR' and the somewhat
    inconsistent, sporadic handling of negatives. Here's our error code bits:

    /*
    * Page fault error code bits:
    *
    * bit 0 == 0: no page found 1: protection fault
    * bit 1 == 0: read access 1: write access
    * bit 2 == 0: kernel-mode access 1: user-mode access
    * bit 3 == 1: use of reserved bit detected
    * bit 4 == 1: fault was an instruction fetch
    * bit 5 == 1: protection keys block access
    */
    enum x86_pf_error_code {
    X86_PF_PROT = 1 << 0,
    X86_PF_WRITE = 1 << 1,
    X86_PF_USER = 1 << 2,
    X86_PF_RSVD = 1 << 3,
    X86_PF_INSTR = 1 << 4,
    X86_PF_PK = 1 << 5,
    };

    While not all of these combinations will happen on real hardware, I think
    the message should nevertheless be fixed length and be of a predictable
    nature.

    I like your '!' idea, but with a further simplification: how about using
    '-/+' differentiation and a single character and a fixed-length message.

    The new output will be lines of:

    #PF error code: -P -W -U -S -I -K (0x00)
    ...
    #PF error code: -P -W +U +S -I -K (0x0c)
    ...
    #PF error code: +P +W +U +S +I +K (0x3f)

    The symbol abbreviations are pretty self-explanatory:

    P = protection fault (X86_PF_PROT)
    W = write access (X86_PF_WRITE)
    U = user-mode access (X86_PF_USER)
    S = supervisor mode (X86_PF_RSVD)
    I = instruction fault (X86_PF_INSTR)
    K = keys fault (X86_PF_PK)

    Misc notes:

    - In principle the new text is now short enough to include it in one of
    the existing output lines, further shortening the oops output - but I
    havent done that in this patch.

    - Another question is the ordering of the bits: the symbolic display is
    actually big endian, while the numeric hexa printout is little endian.

    I kind of still like it that way, not just because the decoding loop is
    more natural, but because the bits are actually ordered by importance:
    the PROT bits is more important than the INSTR or the PK bits - and the
    more important bits are displayed first.

    - Only build-tested the patch and looked at the generated assembly, but
    it all looks sane enough so will obviously work just fine! ;-)

    Thanks,

    Ingo

    ======================>
    Subject: x86/mm/fault: Streamline the fault error_code decoder some more
    From: Ingo Molnar <mingo@kernel.org>
    Date: Thu, 6 Dec 2018 08:12:06 +0100

    Sean Christopherson pointed out that the newfangled human-readable page
    fault oops error_code decoder we added in:

    a1a371c468f7: ("x86/fault: Decode page fault OOPSes better")
    a2aa52ab16ef: ("x86/fault: Clean up the page fault oops decoder a bit")

    *still* confuses humans due to the hiding of the negative case and due to the
    special casing of the all-zeroes error code, which is suboptimal.

    Improve it some more:

    - Change the text from variable-length string to a fixed-length string,

    - display non-set bits,

    - include the error code itself as well numerically,

    - get rid of the '[normal kernel read fault]' special case,

    - factor out the code, simplify and speed up the string generation logic.

    The new output will be lines of:

    #PF error code: -P -W -U -S -I -K (0x00)
    ...
    #PF error code: -P -W +U +S -I -K (0x0c)
    ...
    #PF error code: +P +W +U +S +I +K (0x3f)

    The symbol abbreviations are pretty self-explanatory:

    P = protection fault (X86_PF_PROT)
    W = write access (X86_PF_WRITE)
    U = user-mode access (X86_PF_USER)
    S = supervisor mode (X86_PF_RSVD)
    I = instruction fault (X86_PF_INSTR)
    K = keys fault (X86_PF_PK)

    In principle this is now short enough to include it in one of the
    existing output lines, further shortening the oops output.

    ( Also clean up some nearby line breaks while at it. )

    Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rik van Riel <riel@surriel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/mm/fault.c | 67 +++++++++++++++++++++++++++++++---------------------
    1 file changed, 40 insertions(+), 27 deletions(-)

    Index: tip/arch/x86/mm/fault.c
    ===================================================================
    --- tip.orig/arch/x86/mm/fault.c
    +++ tip/arch/x86/mm/fault.c
    @@ -604,23 +604,51 @@ static void show_ldttss(const struct des
    }

    /*
    - * This helper function transforms the #PF error_code bits into
    - * "[PROT] [USER]" type of descriptive, almost human-readable error strings:
    + * This maps the somewhat obscure error_code number to symbolic text:
    + *
    + * P = protection fault (X86_PF_PROT)
    + * W = write access (X86_PF_WRITE)
    + * U = user-mode access (X86_PF_USER)
    + * S = supervisor mode (X86_PF_RSVD)
    + * I = instruction fault (X86_PF_INSTR)
    + * K = keys fault (X86_PF_PK)
    */
    -static void err_str_append(unsigned long error_code, char *buf, unsigned long mask, const char *txt)
    +static const char error_code_chars[] = "PWUSIK";
    +
    +/*
    + * This helper function transforms the #PF error_code bits into " +P -W +U -R -I -K"
    + * type of descriptive, almost human-readable error strings:
    + */
    +static void show_error_code(struct pt_regs *regs, unsigned long error_code)
    {
    - if (error_code & mask) {
    - if (buf[0])
    - strcat(buf, " ");
    - strcat(buf, txt);
    + unsigned int bit, mask;
    + char err_txt[6*3+1]; /* Fixed length of 6 bits decoded plus zero at the end */
    +
    + /* We go from the X86_PF_PROT bit to the X86_PF_PK bit: */
    +
    + for (bit = 0; bit < 6; bit++) {
    + unsigned int offset = bit*3;
    +
    + err_txt[offset+0] = ' ';
    +
    + mask = 1 << bit;
    + if (error_code & mask)
    + err_txt[offset+1] = '+';
    + else
    + err_txt[offset+1] = '-';
    +
    + err_txt[offset+2] = error_code_chars[bit];
    }
    +
    + /* Close the string: */
    + err_txt[sizeof(err_txt)-1] = 0;
    +
    + pr_alert("#PF error code: %s (%02lx)\n", err_txt, error_code);
    }

    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;

    @@ -648,20 +676,7 @@ show_fault_oops(struct pt_regs *regs, un
    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]");
    + show_error_code(regs, error_code);

    if (!(error_code & X86_PF_USER) && user_mode(regs)) {
    struct desc_ptr idt, gdt;
    @@ -698,8 +713,7 @@ show_fault_oops(struct pt_regs *regs, un
    }

    static noinline void
    -pgtable_bad(struct pt_regs *regs, unsigned long error_code,
    - unsigned long address)
    +pgtable_bad(struct pt_regs *regs, unsigned long error_code, unsigned long address)
    {
    struct task_struct *tsk;
    unsigned long flags;
    @@ -719,8 +733,7 @@ pgtable_bad(struct pt_regs *regs, unsign
    oops_end(flags, regs, sig);
    }

    -static void set_signal_archinfo(unsigned long address,
    - unsigned long error_code)
    +static void set_signal_archinfo(unsigned long address, unsigned long error_code)
    {
    struct task_struct *tsk = current;

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      Last update: 2018-12-06 08:34    [W:2.589 / U:0.196 seconds]
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