Messages in this thread Patch in this message | | | From | Ian Rogers <> | Date | Fri, 29 Jul 2022 22:13:04 -0700 | Subject | Re: [PATCH v3 1/2] perf symbol: Correct address for bss symbols |
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On Sat, Jul 23, 2022 at 11:00 PM Leo Yan <leo.yan@linaro.org> wrote: > > When using 'perf mem' and 'perf c2c', an issue is observed that tool > reports the wrong offset for global data symbols. This is a common > issue on both x86 and Arm64 platforms. > > Let's see an example, for a test program, below is the disassembly for > its .bss section which is dumped with objdump: > > ... > > Disassembly of section .bss: > > 0000000000004040 <completed.0>: > ... > > 0000000000004080 <buf1>: > ... > > 00000000000040c0 <buf2>: > ... > > 0000000000004100 <thread>: > ... > > First we used 'perf mem record' to run the test program and then used > 'perf --debug verbose=4 mem report' to observe what's the symbol info > for 'buf1' and 'buf2' structures. > > # ./perf mem record -e ldlat-loads,ldlat-stores -- false_sharing.exe 8 > # ./perf --debug verbose=4 mem report > ... > dso__load_sym_internal: adjusting symbol: st_value: 0x40c0 sh_addr: 0x4040 sh_offset: 0x3028 > symbol__new: buf2 0x30a8-0x30e8 > ... > dso__load_sym_internal: adjusting symbol: st_value: 0x4080 sh_addr: 0x4040 sh_offset: 0x3028 > symbol__new: buf1 0x3068-0x30a8 > ... > > Perf tool relies on libelf to parse symbols, in executable and shared > object files, 'st_value' holds a virtual address; 'sh_addr' is the > address at which section's first byte should reside in memory, and > 'sh_offset' is the byte offset from the beginning of the file to the > first byte in the section. Perf tool uses below formula to convert > symbol's memory address to file address: > > file_address = st_value - sh_addr + sh_offset > ^ > ` Memory address > > We can see the final adjusted address ranges for buf1 and buf2 are > [0x30a8-0x30e8) and [0x3068-0x30a8) respectively, apparently this is > incorrect, in the code, the structure for 'buf1' and 'buf2' specifies > compiler attribute with 64-byte alignment. > > The problem happens for 'sh_offset', libelf returns it as 0x3028 which > is not 64-byte aligned, combining with disassembly, it's likely libelf > doesn't respect the alignment for .bss section, therefore, it doesn't > return the aligned value for 'sh_offset'. > > Suggested by Fangrui Song, elf file contains program header which > contains PT_LOAD segments, the fields p_vaddr and p_offset in PT_LOAD > segments contain the execution info. A better choice for converting > memory address to file address is using the formula: > > file_address = st_value - p_vaddr + p_offset > > This patch introduces function elf_read_program_header() which returns > out the program header based on the passed 'st_value', then it uses > above formula to calculate the symbol file address; and the debugging > log is updated respectively. > > After applying the change: > > # ./perf --debug verbose=4 mem report > ... > dso__load_sym_internal: adjusting symbol: st_value: 0x40c0 p_vaddr: 0x3d28 p_offset: 0x2d28 > symbol__new: buf2 0x30c0-0x3100 > ... > dso__load_sym_internal: adjusting symbol: st_value: 0x4080 p_vaddr: 0x3d28 p_offset: 0x2d28 > symbol__new: buf1 0x3080-0x30c0 > ... > > Fixes: f17e04afaff8 ("perf report: Fix ELF symbol parsing") > Reported-by: Chang Rui <changruinj@gmail.com> > Suggested-by: Fangrui Song <maskray@google.com> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
I am seeing a problem with this patch with jvmti. To repro:
1) download a Java workload dacapo-9.12-MR1-bach.jar from https://sourceforge.net/projects/dacapobench/ 2) build perf such as "make -C tools/perf O=/tmp/perf NO_LIBBFD=1" it should detect Java and create /tmp/perf/libperf-jvmti.so 3) run perf with the jvmti agent: /tmp/perf/perf record -k 1 java -agentpath:/tmp/perf/libperf-jvmti.so -jar dacapo-9.12-MR1-bach.jar -n 10 fop 4) run perf inject: /tmp/perf/perf inject -i perf.data -o perf-injected.data -j 5) run perf report /tmp/perf/perf report -i perf-injected.data | grep org.apache.fop
With this patch reverted I see lots of symbols like: 0.00% java jitted-388040-4656.so [.] org.apache.fop.fo.FObj.bind(org.apache.fop.fo.PropertyList)
With the patch I see lots of: dso__load_sym_internal: failed to find program header for symbol: Lorg/apache/fop/fo/FObj;bind(Lorg/apache/fop/fo/PropertyList;)V st_value: 0x40
> --- > tools/perf/util/symbol-elf.c | 45 ++++++++++++++++++++++++++++++++---- > 1 file changed, 41 insertions(+), 4 deletions(-) > > diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c > index ecd377938eea..ef6ced5c5746 100644 > --- a/tools/perf/util/symbol-elf.c > +++ b/tools/perf/util/symbol-elf.c > @@ -233,6 +233,33 @@ Elf_Scn *elf_section_by_name(Elf *elf, GElf_Ehdr *ep, > return NULL; > } > > +static int elf_read_program_header(Elf *elf, u64 vaddr, GElf_Phdr *phdr) > +{ > + size_t i, phdrnum; > + u64 sz; > + > + if (elf_getphdrnum(elf, &phdrnum)) > + return -1; > + > + for (i = 0; i < phdrnum; i++) { > + if (gelf_getphdr(elf, i, phdr) == NULL) > + return -1; > + > + if (phdr->p_type != PT_LOAD) > + continue; > + > + sz = max(phdr->p_memsz, phdr->p_filesz); > + if (!sz) > + continue; > + > + if (vaddr >= phdr->p_vaddr && (vaddr < phdr->p_vaddr + sz)) > + return 0; > + } > + > + /* Not found any valid program header */ > + return -1; > +} > + > static bool want_demangle(bool is_kernel_sym) > { > return is_kernel_sym ? symbol_conf.demangle_kernel : symbol_conf.demangle; > @@ -1209,6 +1236,7 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss, > sym.st_value); > used_opd = true; > } > + > /* > * When loading symbols in a data mapping, ABS symbols (which > * has a value of SHN_ABS in its st_shndx) failed at > @@ -1262,11 +1290,20 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss, > goto out_elf_end; > } else if ((used_opd && runtime_ss->adjust_symbols) || > (!used_opd && syms_ss->adjust_symbols)) { > + GElf_Phdr phdr; > + > + if (elf_read_program_header(syms_ss->elf, > + (u64)sym.st_value, &phdr)) { > + pr_warning("%s: failed to find program header for " > + "symbol: %s st_value: %#" PRIx64 "\n", > + __func__, elf_name, (u64)sym.st_value); > + continue; > + } > pr_debug4("%s: adjusting symbol: st_value: %#" PRIx64 " " > - "sh_addr: %#" PRIx64 " sh_offset: %#" PRIx64 "\n", __func__, > - (u64)sym.st_value, (u64)shdr.sh_addr, > - (u64)shdr.sh_offset); > - sym.st_value -= shdr.sh_addr - shdr.sh_offset; > + "p_vaddr: %#" PRIx64 " p_offset: %#" PRIx64 "\n", > + __func__, (u64)sym.st_value, (u64)phdr.p_vaddr, > + (u64)phdr.p_offset); > + sym.st_value -= phdr.p_vaddr - phdr.p_offset; > }
Combining the old and new behaviors fixes the issue for me, wdyt?
``` --- a/tools/perf/util/symbol-elf.c +++ b/tools/perf/util/symbol-elf.c @@ -1305,16 +1305,21 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms _ss,
if (elf_read_program_header(syms_ss->elf, (u64)sym.st_value, &phdr)) { - pr_warning("%s: failed to find program header for " + pr_debug4("%s: failed to find program header for " "symbol: %s st_value: %#" PRIx64 "\n", __func__, elf_name, (u64)sym.st_value); - continue; + pr_debug4("%s: adjusting symbol: st_value: %#" PRIx64 " " + "sh_addr: %#" PRIx64 " sh_offset: %#" PRIx64 "\n", + __func__, (u64)sym.st_value, (u64)shdr.sh_addr, + (u64)shdr.sh_offset); + sym.st_value -= shdr.sh_addr - shdr.sh_offset; + } else { + pr_debug4("%s: adjusting symbol: st_value: %#" PRIx64 " " + "p_vaddr: %#" PRIx64 " p_offset: %#" PRIx64 "\n", + __func__, (u64)sym.st_value, (u64)phdr.p_vaddr, + (u64)phdr.p_offset); + sym.st_value -= phdr.p_vaddr - phdr.p_offset; } - pr_debug4("%s: adjusting symbol: st_value: %#" PRIx64 " " - "p_vaddr: %#" PRIx64 " p_offset: %#" PRIx64 "\n", - __func__, (u64)sym.st_value, (u64)phdr.p_vaddr, - (u64)phdr.p_offset); - sym.st_value -= phdr.p_vaddr - phdr.p_offset; }
demangled = demangle_sym(dso, kmodule, elf_name); ``` Thanks, Ian
> > demangled = demangle_sym(dso, kmodule, elf_name); > -- > 2.25.1 >
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