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SubjectRe: [PATCH v6 06/21] perf arm-spe: Refactor printing string to buffer
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On 03/11/2020 06:40, Leo Yan wrote:

Hi Dave, Leo,

> On Mon, Nov 02, 2020 at 05:06:53PM +0000, Dave Martin wrote:
>> On Fri, Oct 30, 2020 at 02:57:09AM +0000, Leo Yan wrote:
>>> When outputs strings to the decoding buffer with function snprintf(),
>>> SPE decoder needs to detects if any error returns from snprintf() and if
>>> so needs to directly bail out. If snprintf() returns success, it needs
>>> to update buffer pointer and reduce the buffer length so can continue to
>>> output the next string into the consequent memory space.
>>>
>>> This complex logics are spreading in the function arm_spe_pkt_desc() so
>>> there has many duplicate codes for handling error detecting, increment
>>> buffer pointer and decrement buffer size.
>>>
>>> To avoid the duplicate code, this patch introduces a new helper function
>>> arm_spe_pkt_snprintf() which is used to wrap up the complex logics, and
>>> it's used by the caller arm_spe_pkt_desc(); if printing buffer is called
>>> for multiple times in a flow, the error is a cumulative value and simply
>>> returns its final value.
>>>
>>> This patch also moves the variable 'blen' as the function's local
>>> variable, this allows to remove the unnecessary braces and improve the
>>> readability.
>>>
>>> Suggested-by: Dave Martin <Dave.Martin@arm.com>
>>> Signed-off-by: Leo Yan <leo.yan@linaro.org>
>>
>> This looks like a good refacroting now, but as pointed out by Andre this
>> patch is now rather hard to review, since it combines the refactoring
>> with other changes.
>>
>> If reposting this series, it would be good if this could be split into a
>> first patch that introduces arm_spe_pkt_snprintf() and just updates each
>> snprintf() call site to use it, but without moving other code around or
>> optimising anything, followed by one or more patches that clean up and
>> simplify arm_spe_pkt_desc().
>
> I will respin the patch set and follow this approach.

Well, I am afraid this is not easily possible.

Dave: this patch is basically following the pattern turning this:
===============
if (condition) {
ret = snprintf(buf, buf_len, "foo");
buf += ret;
blen -= ret;
}
...
if (ret < 0)
return ret;
blen -= ret;
return buf_len - blen;
===============
into this:
---------------
if (condition)
arm_spe_pkt_snprintf(&err, &buf, &blen, "foo");
...
return err ?: (int)(buf_len - blen);
---------------

And "diff" is getting really ahead of itself here and tries to be super
clever, which leads to this hard to read patch.

But I don't think there is anything we can really do here, this is
already the minimal version. Leo adds the optimisations only later on,
in other patches.

Cheers,
Andre


>
>> If the series is otherwise mature though, then this rework may be
>> overkill.
>>
>>> ---
>>> .../arm-spe-decoder/arm-spe-pkt-decoder.c | 267 ++++++++----------
>>> 1 file changed, 117 insertions(+), 150 deletions(-)
>>>
>>> diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
>>> index 04fd7fd7c15f..1ecaf9805b79 100644
>>> --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
>>> +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
>>> @@ -9,6 +9,7 @@
>>> #include <endian.h>
>>> #include <byteswap.h>
>>> #include <linux/bitops.h>
>>> +#include <stdarg.h>
>>>
>>> #include "arm-spe-pkt-decoder.h"
>>>
>>> @@ -258,192 +259,158 @@ int arm_spe_get_packet(const unsigned char *buf, size_t len,
>>> return ret;
>>> }
>>>
>>> +static int arm_spe_pkt_snprintf(int *err, char **buf_p, size_t *blen,
>>> + const char *fmt, ...)
>>> +{
>>> + va_list ap;
>>> + int ret;
>>> +
>>> + /* Bail out if any error occurred */
>>> + if (err && *err)
>>> + return *err;
>>> +
>>> + va_start(ap, fmt);
>>> + ret = vsnprintf(*buf_p, *blen, fmt, ap);
>>> + va_end(ap);
>>> +
>>> + if (ret < 0) {
>>> + if (err && !*err)
>>> + *err = ret;
>>
>> What happens on buffer overrun (i.e., ret >= *blen)?
>>
>> It looks to me like we'll advance buf_p too far, blen will wrap around,
>> and the string at *buf_p won't be null terminated. Because the return
>> value is still >= 0, this condition will be returned up the stack as
>> "success".
>
> Thanks for pointint out this. I never note for the potential issue
> caused by returned value (ret >= *blen); checked again for the
> manual, it says:
>
> "The functions snprintf() and vsnprintf() do not write more than size
> bytes (including the terminating null byte ('\0')). If the output was
> truncated due to this limit, then the return value is the number of
> characters (excluding the terminating null byte) which would have
> been written to the final string if enough space had been available.
> Thus, a return value of size or more means that the output was
> truncated."
>
>> Perhaps this can never happen given the actual buffer sizes and strings
>> being printed, but it feels a bit unsafe.
>>
>>
>> It may be better to clamp the adjustments to *buf_p and *blen to
>> *blen - 1 in this case, and explicitly set (*buf_p)[*blen - 1] to '\0'.
>> We _may_ want indicate failure in the return from arm_spe_pkt_desc() in
>> this situation, but I don't know enough about how this code is called to
>> enable me to judge that.
>
> The caller arm_spe_dump() will print out the string if the return
> value > 0 [1]; so I think it can simply return the string length which
> has been written to the buffer (with the clamped value). The function
> can be refined as below:
>
> static int arm_spe_pkt_snprintf(int *err, char **buf_p, size_t *blen,
> const char *fmt, ...)
> {
> va_list ap;
> int ret;
>
> /* Bail out if any error occurred */
> if (err && *err)
> return *err;
>
> va_start(ap, fmt);
> ret = vsnprintf(*buf_p, *blen - 1, fmt, ap);
> va_end(ap);
>
> if (ret < 0) {
> if (err && !*err)
> *err = ret;
> } else {
>
> /*
> * A return value of (*blen - 1) or more means that the
> * output was truncated and the buffer is overrun.
> */
> if (ret >= (*blen - 1)) {
> (*buf_p)[*blen - 1] = '\0';
>
> /*
> * Set *err to -1 to avoid overflow if tries to
> * fill this buffer sequentially.
> */
> if (err && !*err)
> *err = -1;
>
> /* Clamp the size of characters printed */
> ret = min(ret, *blen);
> }
>
> *buf_p += ret;
> *blen -= ret;
> }
>
> return ret;
> }
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/arm-spe.c#n116
>
>> (Note, this issue is not introduced by this patch, but this refactoring
>> makes it easier to address it in a single place -- so it may now be
>> worth doing so.)
>
> Agree.
>
>>> + } else {
>>> + *buf_p += ret;
>>> + *blen -= ret;
>>> + }
>>> +
>>> + return ret;
>>> +}
>>> +
>>> int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
>>> size_t buf_len)
>>> {
>>> - int ret, ns, el, idx = packet->index;
>>> + int ns, el, idx = packet->index;
>>> unsigned long long payload = packet->payload;
>>> const char *name = arm_spe_pkt_name(packet->type);
>>> + size_t blen = buf_len;
>>> + int err = 0;
>>>
>>> switch (packet->type) {
>>> case ARM_SPE_BAD:
>>> case ARM_SPE_PAD:
>>> case ARM_SPE_END:
>>> - return snprintf(buf, buf_len, "%s", name);
>>> - case ARM_SPE_EVENTS: {
>>> - size_t blen = buf_len;
>>> -
>>> - ret = 0;
>>> - ret = snprintf(buf, buf_len, "EV");
>>> - buf += ret;
>>> - blen -= ret;
>>> - if (payload & 0x1) {
>>> - ret = snprintf(buf, buf_len, " EXCEPTION-GEN");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x2) {
>>> - ret = snprintf(buf, buf_len, " RETIRED");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x4) {
>>> - ret = snprintf(buf, buf_len, " L1D-ACCESS");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x8) {
>>> - ret = snprintf(buf, buf_len, " L1D-REFILL");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x10) {
>>> - ret = snprintf(buf, buf_len, " TLB-ACCESS");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x20) {
>>> - ret = snprintf(buf, buf_len, " TLB-REFILL");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x40) {
>>> - ret = snprintf(buf, buf_len, " NOT-TAKEN");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x80) {
>>> - ret = snprintf(buf, buf_len, " MISPRED");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen, "%s", name);
>>> + case ARM_SPE_EVENTS:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, "EV");
>>> +
>>> + if (payload & 0x1)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " EXCEPTION-GEN");
>>> + if (payload & 0x2)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " RETIRED");
>>> + if (payload & 0x4)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " L1D-ACCESS");
>>> + if (payload & 0x8)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " L1D-REFILL");
>>> + if (payload & 0x10)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " TLB-ACCESS");
>>> + if (payload & 0x20)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " TLB-REFILL");
>>> + if (payload & 0x40)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " NOT-TAKEN");
>>> + if (payload & 0x80)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " MISPRED");
>>> if (idx > 1) {
>>> - if (payload & 0x100) {
>>> - ret = snprintf(buf, buf_len, " LLC-ACCESS");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x200) {
>>> - ret = snprintf(buf, buf_len, " LLC-REFILL");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x400) {
>>> - ret = snprintf(buf, buf_len, " REMOTE-ACCESS");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> + if (payload & 0x100)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " LLC-ACCESS");
>>> + if (payload & 0x200)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " LLC-REFILL");
>>> + if (payload & 0x400)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " REMOTE-ACCESS");
>>> }
>>> - if (ret < 0)
>>> - return ret;
>>> - blen -= ret;
>>> - return buf_len - blen;
>>
>> It looks like we now fall off the bottom of the switch() here. It's
>> preferable to add a break, both to document the intended code flow, and
>> to avoid accidents if another case is added later.
>
> Will do this.
>
>>> - }
>>> + return err ?: (int)(buf_len - blen);
>>
>> Nit: unexplained type cast. Does the result definitely fit into an int?
>> If not, why doesn't it matter?
>
> The case is to dismiss the compiler warning.
>
> GCC doesn't compliant for the code:
>
> return buf_len - blen;
>
> But GCC compliants type mismatch after change to code:
>
> return err ?: buf_len - blen;
>
>> Also:
>>
>> If the actual return value is important for the caller to determine the
>> number of bytes appended, then it would be better to compute it in one
>> place. Otherwise, it would be better to squash all success returns to 0,
>> rather than spend effort computing a value that may be misleading or
>> wrong anyway.
>>
>> In its current form, it's tricky to see whether this patch derives the
>> return value consistently for all cases. In particular, if some code
>> patch does one or more arm_spe_pkt_snprintf(), followed by a
>> return arm_spe_pkt_snprintf(); then the return value (which only takes
>> the last arm_spe_pkt_desc() into account) would be wrong. It would be
>> preferable if the return value for the success case were always
>> computed from buf_len and blen, to avoid this risk. (I'm not saying
>> this bug exists, just that it's hard to see from the patch that it
>> doesn't exist.)
>
> I think I can go back to return value "buf_len - blen" and don't
> check "err" anymore.
>
> If the string buffer is truncated due to size limitation, we can still
> return "buf_len - blen" so can allow caller to dump string as
> possible.
>
> If arm_spe_pkt_snprintf() returns -1 at the first call, "buf_len" is
> equal to "blen", thus return value of "buf_len - blen" will be zero.
> For this case, the caller will get to know there have no valid string
> and will skip to print anything.
>
> Does this make sense?
>
>>> +
>>> case ARM_SPE_OP_TYPE:
>>> switch (idx) {
>>> - case 0: return snprintf(buf, buf_len, "%s", payload & 0x1 ?
>>> - "COND-SELECT" : "INSN-OTHER");
>>> - case 1: {
>>> - size_t blen = buf_len;
>>> + case 0:
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen,
>>> + payload & 0x1 ? "COND-SELECT" : "INSN-OTHER");
>>> + case 1:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen,
>>> + payload & 0x1 ? "ST" : "LD");
>>>
>>> - if (payload & 0x1)
>>> - ret = snprintf(buf, buf_len, "ST");
>>> - else
>>> - ret = snprintf(buf, buf_len, "LD");
>>> - buf += ret;
>>> - blen -= ret;
>>> if (payload & 0x2) {
>>> - if (payload & 0x4) {
>>> - ret = snprintf(buf, buf_len, " AT");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x8) {
>>> - ret = snprintf(buf, buf_len, " EXCL");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x10) {
>>> - ret = snprintf(buf, buf_len, " AR");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> + if (payload & 0x4)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " AT");
>>> + if (payload & 0x8)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " EXCL");
>>> + if (payload & 0x10)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " AR");
>>> } else if (payload & 0x4) {
>>> - ret = snprintf(buf, buf_len, " SIMD-FP");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (ret < 0)
>>> - return ret;
>>> - blen -= ret;
>>> - return buf_len - blen;
>>> - }
>>> - case 2: {
>>> - size_t blen = buf_len;
>>> -
>>> - ret = snprintf(buf, buf_len, "B");
>>> - buf += ret;
>>> - blen -= ret;
>>> - if (payload & 0x1) {
>>> - ret = snprintf(buf, buf_len, " COND");
>>> - buf += ret;
>>> - blen -= ret;
>>> - }
>>> - if (payload & 0x2) {
>>> - ret = snprintf(buf, buf_len, " IND");
>>> - buf += ret;
>>> - blen -= ret;
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " SIMD-FP");
>>> }
>>> - if (ret < 0)
>>> - return ret;
>>> - blen -= ret;
>>> - return buf_len - blen;
>>> - }
>>> - default: return 0;
>>> +
>>> + return err ?: (int)(buf_len - blen);
>>> +
>>> + case 2:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, "B");
>>> +
>>> + if (payload & 0x1)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " COND");
>>> + if (payload & 0x2)
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, " IND");
>>> +
>>> + return err ?: (int)(buf_len - blen);
>>> +
>>> + default:
>>> + return 0;
>>> }
>>> case ARM_SPE_DATA_SOURCE:
>>> case ARM_SPE_TIMESTAMP:
>>> - return snprintf(buf, buf_len, "%s %lld", name, payload);
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen, "%s %lld", name, payload);
>>> case ARM_SPE_ADDRESS:
>>> switch (idx) {
>>> case 0:
>>> case 1: ns = !!(packet->payload & NS_FLAG);
>>> el = (packet->payload & EL_FLAG) >> 61;
>>> payload &= ~(0xffULL << 56);
>>> - return snprintf(buf, buf_len, "%s 0x%llx el%d ns=%d",
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen,
>>> + "%s 0x%llx el%d ns=%d",
>>> (idx == 1) ? "TGT" : "PC", payload, el, ns);
>>> - case 2: return snprintf(buf, buf_len, "VA 0x%llx", payload);
>>> + case 2:
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen,
>>> + "VA 0x%llx", payload);
>>> case 3: ns = !!(packet->payload & NS_FLAG);
>>> payload &= ~(0xffULL << 56);
>>> - return snprintf(buf, buf_len, "PA 0x%llx ns=%d",
>>> - payload, ns);
>>> - default: return 0;
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen,
>>> + "PA 0x%llx ns=%d", payload, ns);
>>> + default:
>>> + return 0;
>>> }
>>> case ARM_SPE_CONTEXT:
>>> - return snprintf(buf, buf_len, "%s 0x%lx el%d", name,
>>> - (unsigned long)payload, idx + 1);
>>> - case ARM_SPE_COUNTER: {
>>> - size_t blen = buf_len;
>>> -
>>> - ret = snprintf(buf, buf_len, "%s %d ", name,
>>> - (unsigned short)payload);
>>> - buf += ret;
>>> - blen -= ret;
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen, "%s 0x%lx el%d",
>>> + name, (unsigned long)payload, idx + 1);
>>> + case ARM_SPE_COUNTER:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, "%s %d ", name,
>>> + (unsigned short)payload);
>>> +
>>> switch (idx) {
>>> - case 0: ret = snprintf(buf, buf_len, "TOT"); break;
>>> - case 1: ret = snprintf(buf, buf_len, "ISSUE"); break;
>>> - case 2: ret = snprintf(buf, buf_len, "XLAT"); break;
>>> - default: ret = 0;
>>> + case 0:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, "TOT");
>>> + break;
>>> + case 1:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, "ISSUE");
>>> + break;
>>> + case 2:
>>> + arm_spe_pkt_snprintf(&err, &buf, &blen, "XLAT");
>>> + break;
>>> + default:
>>> + break;
>>> }
>>> - if (ret < 0)
>>> - return ret;
>>> - blen -= ret;
>>> - return buf_len - blen;
>>> - }
>>> +
>>> + return err ?: (int)(buf_len - blen);
>>> +
>>> default:
>>> break;
>>> }
>>>
>>> - return snprintf(buf, buf_len, "%s 0x%llx (%d)",
>>> - name, payload, packet->index);
>>> + return arm_spe_pkt_snprintf(&err, &buf, &blen, "%s 0x%llx (%d)",
>>> + name, payload, packet->index);
>>
>> Otherwise, the patch looks generally OK, but I'd like to see an answer
>> on my points above.
>
> Thanks a lot for reviewing!
>
> Leo
>

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