Messages in this thread Patch in this message | | | Subject | Re: [PATCH v6 06/21] perf arm-spe: Refactor printing string to buffer | From | André Przywara <> | Date | Tue, 3 Nov 2020 10:13:49 +0000 |
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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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