elf: skip old version symbols during lookup

This patch modifies dynamic loader lookup logic to make it
skip old symbols which are marked as such per version symbols table.

This gap in symbols lookup logic would affect some executables
that depend on libraries that happen to have multiple symbol
implementation versions. The good example is numactl that depends on
libnuma.so.

One can test this patch by trying to run numactl before and after:
```
./scripts/manifest_from_host.sh -w numactl && ./scripts/build fs=rofs -j4 --append-manifest
./scripts/run.py -e '/numactl --show'
```

For details of ELF symbol versioning, please read https://www.akkadia.org/drepper/symbol-versioning.
This paragraph is most applicable to what this patch addresses:

"If the highest bit (no. 15) of the version symbol value is set, the
object is hidden and must not be used.  In this case the linker must
treat the symbol as not present in the object."

Please note this patch does NOT make OSv dynamic linker fully
implement symbols versioning logic.

Signed-off-by: Waldemar Kozaczuk <[email protected]>
Message-Id: <[email protected]>

core/elf.cc

@@ -789,6 +789,8 @@ elf64_hash(const char *name)
     return h;
 }
 
+constexpr Elf64_Versym old_version_symbol_mask = Elf64_Versym(1) << 15;
+
 Elf64_Sym* object::lookup_symbol_old(const char* name)
 {
     auto symtab = dynamic_ptr<Elf64_Sym>(DT_SYMTAB);
@@ -842,10 +844,14 @@ Elf64_Sym* object::lookup_symbol_gnu(const char* name)
     if (idx == 0) {
         return nullptr;
     }
+    auto version_symtab = dynamic_exists(DT_VERSYM) ? dynamic_ptr<Elf64_Versym>(DT_VERSYM) : nullptr;
     do {
         if ((chains[idx] & ~1) != (hashval & ~1)) {
             continue;
         }
+        if (version_symtab && version_symtab[idx] & old_version_symbol_mask) { //Ignore old version symbols
+            continue;
+        }
         if (strcmp(&strtab[symtab[idx].st_name], name) == 0) {
             return &symtab[idx];
         }

include/osv/elf.hh

@@ -194,6 +194,7 @@ enum {
     DT_RUNPATH = 29, // d_val The string table offset of a shared library search path string.
     DT_FLAGS = 30, // value is various flags, bits from DF_*.
     DT_FLAGS_1 = 0x6ffffffb, // value is various flags, bits from DF_1_*.
+    DT_VERSYM = 0x6ffffff0, // d_ptr Address of the version symbol table.
     DT_LOOS = 0x60000000, // Defines a range of dynamic table tags that are reserved for
       // environment-specific use.
     DT_HIOS = 0x6FFFFFFF, //