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Subject[perf] perf_event_open() sometimes returning 0
Hello

so I was tracking down some odd behavior in the perf_fuzzer which turns
out to be because perf_even_open() sometimes returns 0 (indicating a file
descriptor of 0) even though as far as I can tell stdin is still open.

Before I waste too much time trying to track this down, is this a known
issue?

Some sample strace output:

perf_event_open({type=PERF_TYPE_RAW, size=0x78 /* PERF_ATTR_SIZE_??? */, config=0x1313, ...}, 0, 3, -1, PERF_FLAG_FD_NO_GROUP) = 0
perf_event_open({type=PERF_TYPE_SOFTWARE, size=0x78 /* PERF_ATTR_SIZE_??? */, config=PERF_COUNT_SW_DUMMY, ...}, -1, 3, -1, PERF_FLAG_FD_NO_GROUP|PERF_FLAG_FD_OUTPUT) = 0
perf_event_open({type=PERF_TYPE_SOFTWARE, size=0x78 /* PERF_ATTR_SIZE_??? */, config=PERF_COUNT_SW_PAGE_FAULTS_MIN, ...}, 158266, 2, -1, PERF_FLAG_FD_CLOEXEC) = 0
perf_event_open({type=PERF_TYPE_HW_CACHE, size=0x78 /* PERF_ATTR_SIZE_??? */, config=PERF_COUNT_HW_CACHE_DTLB|PERF_COUNT_HW_CACHE_OP_READ<<8|PERF_COUNT_HW_CACHE_RESULT_MISS<<16, ...}, 0, 4, -1, PERF_FLAG_FD_NO_GROUP|PERF_FLAG_FD_CLOEXEC) = 0
perf_event_open({type=PERF_TYPE_RAW, size=0x78 /* PERF_ATTR_SIZE_??? */, config=0, ...}, -1, 0, -1, PERF_FLAG_FD_OUTPUT|PERF_FLAG_FD_CLOEXEC) = 0

On my Haswell system (running current git) I can reproduce things with a
single call:

memset(&pe[0],0,sizeof(struct perf_event_attr));
pe[0].type=PERF_TYPE_RAW;
pe[0].size=120;
pe[0].config=0x0ULL;
pe[0].sample_period=0x4777c3ULL;
pe[0].sample_type=PERF_SAMPLE_STREAM_ID; /* 200 */
pe[0].read_format=PERF_FORMAT_TOTAL_TIME_RUNNING|PERF_FORMAT_ID|PERF_FORMAT_GROUP; /* e */
pe[0].inherit=1;
pe[0].exclude_hv=1;
pe[0].exclude_idle=1;
pe[0].enable_on_exec=1;
pe[0].watermark=1;
pe[0].precise_ip=0; /* arbitrary skid */
pe[0].mmap_data=1;
pe[0].exclude_guest=1;
pe[0].exclude_callchain_kernel=1;
pe[0].mmap2=1;
pe[0].comm_exec=1;
pe[0].context_switch=1;
pe[0].bpf_event=1;
pe[0].wakeup_watermark=47545;
pe[0].bp_type=HW_BREAKPOINT_EMPTY;
pe[0].branch_sample_type=PERF_SAMPLE_BRANCH_KERNEL|PERF_SAMPLE_BRANCH_ANY_RETURN|PERF_SAMPLE_BRANCH_COND|0x800ULL;
pe[0].sample_regs_user=42ULL;
pe[0].sample_stack_user=0xfffffffd;
pe[0].aux_watermark=25443;
pe[0].aux_sample_size=8192;

fd[0]=perf_event_open(&pe[0],
-1, /* current thread */
0, /* Only cpu 0 */
-1, /* New Group Leader */
PERF_FLAG_FD_OUTPUT|PERF_FLAG_FD_CLOEXEC /*a*/ );

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