The text and data address is saved in the meta data so that it can be used to know the delta of the text and data addresses of the last boot compared to the text and data addresses of the current boot. The delta is used to convert function pointer entries in the ring buffer to something that can be used by kallsyms (note this only works for built-in functions). But the saved addresses get reset on boot up. If the buffer is not used and there's another reboot, then the saved text and data addresses will be of the last boot and not that of the boot that created the content in the ring buffer. To get an idea of the issue: # trace-cmd start -B boot_mapped -p function # reboot # trace-cmd show -B boot_mapped | tail <...>-1 [000] d..1. 461.983243: native_apic_msr_write <-native_kick_ap <...>-1 [000] d..1. 461.983244: __pfx_native_apic_msr_eoi <-native_kick_ap <...>-1 [000] d..1. 461.983244: reserve_irq_vector_locked <-native_kick_ap <...>-1 [000] d..1. 461.983262: branch_emulate_op <-native_kick_ap <...>-1 [000] d..1. 461.983262: __ia32_sys_ia32_pread64 <-native_kick_ap <...>-1 [000] d..1. 461.983263: native_kick_ap <-__smpboot_create_thread <...>-1 [000] d..1. 461.983263: store_cache_disable <-native_kick_ap <...>-1 [000] d..1. 461.983279: acpi_power_off_prepare <-native_kick_ap <...>-1 [000] d..1. 461.983280: __pfx_acpi_ns_delete_node <-acpi_suspend_enter <...>-1 [000] d..1. 461.983280: __pfx_acpi_os_release_lock <-acpi_suspend_enter # reboot # trace-cmd show -B boot_mapped |tail <...>-1 [000] d..1. 461.983243: 0xffffffffa9669220 <-0xffffffffa965f3db <...>-1 [000] d..1. 461.983244: 0xffffffffa96690f0 <-0xffffffffa965f3db <...>-1 [000] d..1. 461.983244: 0xffffffffa9663fa0 <-0xffffffffa965f3db <...>-1 [000] d..1. 461.983262: 0xffffffffa9672e80 <-0xffffffffa965f3e0 <...>-1 [000] d..1. 461.983262: 0xffffffffa962b940 <-0xffffffffa965f3ec <...>-1 [000] d..1. 461.983263: 0xffffffffa965f540 <-0xffffffffa96e1362 <...>-1 [000] d..1. 461.983263: 0xffffffffa963c940 <-0xffffffffa965f55b <...>-1 [000] d..1. 461.983279: 0xffffffffa9ee30c0 <-0xffffffffa965f59b <...>-1 [000] d..1. 461.983280: 0xffffffffa9f16c10 <-0xffffffffa9ee3157 <...>-1 [000] d..1. 461.983280: 0xffffffffa9ee02e0 <-0xffffffffa9ee3157 By not updating the saved text and data addresses in the meta data at every boot up and only updating them when the buffer is reset, it allows multiple boots to see the same data.
Showing the changes to kernel/trace/ring_buffer.c (1 of 1 file). Show all
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| @@ -1817,12 +1817,19 @@ static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer) · 1816 unmodified lines | ||
| 1817 | 1817 | /* Used to calculate data delta */ |
| 1818 | 1818 | static char rb_data_ptr[] = ""; |
| 1819 | 1819 | |
| 1820 | +#define THIS_TEXT_PTR ((unsigned long)rb_meta_init_text_addr) | |
| 1821 | +#define THIS_DATA_PTR ((unsigned long)rb_data_ptr) | |
| 1822 | + | |
| 1823 | +static void rb_meta_init_text_addr(struct ring_buffer_meta *meta) | |
| 1824 | +{ | |
| 1825 | + meta->text_addr = THIS_TEXT_PTR; | |
| 1826 | + meta->data_addr = THIS_DATA_PTR; | |
| 1827 | +} | |
| 1828 | + | |
| 1820 | 1829 | static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages) |
| 1821 | 1830 | { |
| 1822 | 1831 | struct ring_buffer_meta *meta; |
| 1823 | 1832 | unsigned long delta; |
| 1824 | − unsigned long this_text = (unsigned long)rb_range_meta_init; | |
| 1825 | − unsigned long this_data = (unsigned long)rb_data_ptr; | |
| 1826 | 1833 | void *subbuf; |
| 1827 | 1834 | int cpu; |
| 1828 | 1835 | int i; |
| @@ -1839,10 +1846,8 @@ static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages) · 10 unmodified lines | ||
| 1839 | 1846 | meta->first_buffer += delta; |
| 1840 | 1847 | meta->head_buffer += delta; |
| 1841 | 1848 | meta->commit_buffer += delta; |
| 1842 | − buffer->last_text_delta = this_text - meta->text_addr; | |
| 1843 | − buffer->last_data_delta = this_data - meta->data_addr; | |
| 1844 | − meta->text_addr = this_text; | |
| 1845 | − meta->data_addr = this_data; | |
| 1849 | + buffer->last_text_delta = THIS_TEXT_PTR - meta->text_addr; | |
| 1850 | + buffer->last_data_delta = THIS_DATA_PTR - meta->data_addr; | |
| 1846 | 1851 | continue; |
| 1847 | 1852 | } |
| 1848 | 1853 | |
| @@ -1859,8 +1864,7 @@ static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages) · 10 unmodified lines | ||
| 1859 | 1864 | subbuf = rb_subbufs_from_meta(meta); |
| 1860 | 1865 | |
| 1861 | 1866 | meta->first_buffer = (unsigned long)subbuf; |
| 1862 | − meta->text_addr = this_text; | |
| 1863 | − meta->data_addr = this_data; | |
| 1867 | + rb_meta_init_text_addr(meta); | |
| 1864 | 1868 | |
| 1865 | 1869 | /* |
| 1866 | 1870 | * The buffers[] array holds the order of the sub-buffers |
| @@ -5990,6 +5994,7 @@ static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer) · 4123 unmodified lines | ||
| 5990 | 5994 | void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu) |
| 5991 | 5995 | { |
| 5992 | 5996 | struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu]; |
| 5997 | + struct ring_buffer_meta *meta; | |
| 5993 | 5998 | |
| 5994 | 5999 | if (!cpumask_test_cpu(cpu, buffer->cpumask)) |
| 5995 | 6000 | return; |
| @@ -6008,6 +6013,11 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu) · 12 unmodified lines | ||
| 6008 | 6013 | atomic_dec(&cpu_buffer->record_disabled); |
| 6009 | 6014 | atomic_dec(&cpu_buffer->resize_disabled); |
| 6010 | 6015 | |
| 6016 | + /* Make sure persistent meta now uses this buffer's addresses */ | |
| 6017 | + meta = rb_range_meta(buffer, 0, cpu_buffer->cpu); | |
| 6018 | + if (meta) | |
| 6019 | + rb_meta_init_text_addr(meta); | |
| 6020 | + | |
| 6011 | 6021 | mutex_unlock(&buffer->mutex); |
| 6012 | 6022 | } |
| 6013 | 6023 | EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); |
| @@ -6022,6 +6032,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); · 8 unmodified lines | ||
| 6022 | 6032 | void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) |
| 6023 | 6033 | { |
| 6024 | 6034 | struct ring_buffer_per_cpu *cpu_buffer; |
| 6035 | + struct ring_buffer_meta *meta; | |
| 6025 | 6036 | int cpu; |
| 6026 | 6037 | |
| 6027 | 6038 | /* prevent another thread from changing buffer sizes */ |
| @@ -6049,6 +6060,11 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) · 21 unmodified lines | ||
| 6049 | 6060 | |
| 6050 | 6061 | reset_disabled_cpu_buffer(cpu_buffer); |
| 6051 | 6062 | |
| 6063 | + /* Make sure persistent meta now uses this buffer's addresses */ | |
| 6064 | + meta = rb_range_meta(buffer, 0, cpu_buffer->cpu); | |
| 6065 | + if (meta) | |
| 6066 | + rb_meta_init_text_addr(meta); | |
| 6067 | + | |
| 6052 | 6068 | atomic_dec(&cpu_buffer->record_disabled); |
| 6053 | 6069 | atomic_sub(RESET_BIT, &cpu_buffer->resize_disabled); |
| 6054 | 6070 | } |