perf symbols: Allow for .plt entries with no symbol

Create a sensible name for .plt entries with no symbol.

Example:

 Before:

   $ perf test --dso /usr/lib/x86_64-linux-gnu/libc.so.6 -vv Symbols 2>/tmp/cmp1.txt

 After:

   $ perf test --dso /usr/lib/x86_64-linux-gnu/libc.so.6 -vv Symbols 2>/tmp/cmp2.txt
   $ diff /tmp/cmp1.txt /tmp/cmp2.txt
    4c4
    < test child forked, pid 53043
    ---
    > test child forked, pid 54372
    23,62c23,62
    <  280f0-28100 g @plt
    <  28100-28110 g @plt
    <  28110-28120 g @plt
    <  28120-28130 g @plt
    <  28130-28140 g @plt
    <  28140-28150 g @plt
    <  28150-28160 g @plt
    <  28160-28170 g @plt
    <  28170-28180 g @plt
    <  28180-28190 g @plt
    <  28190-281a0 g @plt
    <  281a0-281b0 g @plt
    <  281b0-281c0 g @plt
    <  281c0-281d0 g @plt
    <  281d0-281e0 g @plt
    <  281e0-281f0 g @plt
    <  281f0-28200 g @plt
    <  28200-28210 g @plt
    <  28210-28220 g @plt
    <  28220-28230 g @plt
    <  28230-28240 g @plt
    <  28240-28250 g @plt
    <  28250-28260 g @plt
    <  28260-28270 g @plt
    <  28270-28280 g @plt
    <  28280-28290 g @plt
    <  28290-282a0 g @plt
    <  282a0-282b0 g @plt
    <  282b0-282c0 g @plt
    <  282c0-282d0 g @plt
    <  282d0-282e0 g @plt
    <  282e0-282f0 g @plt
    <  282f0-28300 g @plt
    <  28300-28310 g @plt
    <  28310-28320 g @plt
    <  28320-28330 g @plt
    <  28330-28340 g @plt
    <  28340-28350 g @plt
    <  28350-28360 g @plt
    <  28360-28370 g @plt
    ---
    >  280f0-28100 g offset_0x280f0@plt
    >  28100-28110 g offset_0x28100@plt
    >  28110-28120 g offset_0x28110@plt
    >  28120-28130 g offset_0x28120@plt
    >  28130-28140 g offset_0x28130@plt
    >  28140-28150 g offset_0x28140@plt
    >  28150-28160 g offset_0x28150@plt
    >  28160-28170 g offset_0x28160@plt
    >  28170-28180 g offset_0x28170@plt
    >  28180-28190 g offset_0x28180@plt
    >  28190-281a0 g offset_0x28190@plt
    >  281a0-281b0 g offset_0x281a0@plt
    >  281b0-281c0 g offset_0x281b0@plt
    >  281c0-281d0 g offset_0x281c0@plt
    >  281d0-281e0 g offset_0x281d0@plt
    >  281e0-281f0 g offset_0x281e0@plt
    >  281f0-28200 g offset_0x281f0@plt
    >  28200-28210 g offset_0x28200@plt
    >  28210-28220 g offset_0x28210@plt
    >  28220-28230 g offset_0x28220@plt
    >  28230-28240 g offset_0x28230@plt
    >  28240-28250 g offset_0x28240@plt
    >  28250-28260 g offset_0x28250@plt
    >  28260-28270 g offset_0x28260@plt
    >  28270-28280 g offset_0x28270@plt
    >  28280-28290 g offset_0x28280@plt
    >  28290-282a0 g offset_0x28290@plt
    >  282a0-282b0 g offset_0x282a0@plt
    >  282b0-282c0 g offset_0x282b0@plt
    >  282c0-282d0 g offset_0x282c0@plt
    >  282d0-282e0 g offset_0x282d0@plt
    >  282e0-282f0 g offset_0x282e0@plt
    >  282f0-28300 g offset_0x282f0@plt
    >  28300-28310 g offset_0x28300@plt
    >  28310-28320 g offset_0x28310@plt
    >  28320-28330 g offset_0x28320@plt
    >  28330-28340 g offset_0x28330@plt
    >  28340-28350 g offset_0x28340@plt
    >  28350-28360 g offset_0x28350@plt
    >  28360-28370 g offset_0x28360@plt
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20230120123456.12449-9-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/symbol-elf.c

@@ -470,8 +470,11 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 			demangled = demangle_sym(dso, 0, elf_name);
 			if (demangled != NULL)
 				elf_name = demangled;
-			snprintf(sympltname, sizeof(sympltname),
-				 "%s@plt", elf_name);
+			if (*elf_name)
+				snprintf(sympltname, sizeof(sympltname), "%s@plt", elf_name);
+			else
+				snprintf(sympltname, sizeof(sympltname),
+					 "offset_%#" PRIx64 "@plt", plt_offset);
 			free(demangled);
 
 			f = symbol__new(plt_offset, plt_entry_size,
@@ -496,8 +499,11 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 			demangled = demangle_sym(dso, 0, elf_name);
 			if (demangled != NULL)
 				elf_name = demangled;
-			snprintf(sympltname, sizeof(sympltname),
-				 "%s@plt", elf_name);
+			if (*elf_name)
+				snprintf(sympltname, sizeof(sympltname), "%s@plt", elf_name);
+			else
+				snprintf(sympltname, sizeof(sympltname),
+					 "offset_%#" PRIx64 "@plt", plt_offset);
 			free(demangled);
 
 			f = symbol__new(plt_offset, plt_entry_size,