debug/elf: validate shstrndx

Changes:

1. When e_shstrndx holds the value SHN_UNDEF (0), the file has no section
name string table. In this case, do not try to set section names .
2. e_shstrndx should point to an SHT_STRTAB section. If it does not, returns
an error.

Reference:
https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.eheader.html

Updates #54967.

Change-Id: Ic8f228061d996fd7845dfa630719a1ba12d2bb60
GitHub-Last-Rev: aeb70ca
GitHub-Pull-Request: #55001
Reviewed-on: https://go-review.googlesource.com/c/go/+/430155
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: ZekeLu

src/debug/elf/file.go

@@ -474,7 +474,16 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	}
 
 	// Load section header string table.
-	shstrtab, err := f.Sections[shstrndx].Data()
+	if shstrndx == 0 {
+		// If the file has no section name string table,
+		// shstrndx holds the value SHN_UNDEF (0).
+		return f, nil
+	}
+	shstr := f.Sections[shstrndx]
+	if shstr.Type != SHT_STRTAB {
+		return nil, &FormatError{shoff + int64(shstrndx*shentsize), "invalid ELF section name string table type", shstr.Type}
+	}
+	shstrtab, err := shstr.Data()
 	if err != nil {
 		return nil, err
 	}