Check for a branded ELF note when OS/ABI is NONE

Currently the auditor is only checking for the correct OS/ABI and
producing a warning on FreeBSD when it doesn't match. This warning is
incorrect on AArch64 though, as FreeBSD instead uses an ELF note to
declare the OS/ABI. The change implemented here looks for such a note
when the OS/ABI in the ELF header is NONE.

src/auditor/dynamic_linkage.jl

@@ -1,5 +1,30 @@
 using ObjectFile.ELF
 
+function os_from_elf_note(oh::ELFHandle)
+    for section in Sections(oh)
+        section_handle(section) == ELF.SHT_NOTE || continue
+        seek(oh, section_offset(section))
+        name_length = read(oh, UInt32)
+        iszero(name_length) && continue
+        descriptor_length = read(oh, UInt32)
+        note_type = read(oh, UInt32)
+        name = String(read(oh, name_length - 1))  # skip trailing NUL
+        if note_type == 1
+            # Technically it's part of the Linux specification that any executable should
+            # have an ELF note with type 1, name GNU, and descriptor length ≥4, but in
+            # practice I haven't observed that consistently, especially on musl. So for
+            # now, only bother checking FreeBSD, which uses an ELF note rather than OS/ABI
+            # to identify itself on AArch64 and RISC-V.
+            if name == "FreeBSD" && descriptor_length == 4
+                return name
+            end
+        end
+    end
+    return nothing
+end
+
+os_from_elf_note(::ObjectHandle) = nothing
+
 """
     platform_for_object(oh::ObjectHandle)
 
@@ -39,7 +64,9 @@ function platform_for_object(oh::ObjectHandle)
             end
         end
 
-        if oh.ei.osabi == ELF.ELFOSABI_LINUX || oh.ei.osabi == ELF.ELFOSABI_NONE
+        if oh.ei.osabi == ELF.ELFOSABI_NONE
+            return Platform(arch, os_from_elf_note(oh) == "FreeBSD" ? "freebsd" : "linux")
+        elseif oh.ei.osabi == ELF.ELFOSABI_LINUX
             return Platform(arch, "linux")
         elseif oh.ei.osabi == ELF.ELFOSABI_FREEBSD
             return Platform(arch, "freebsd")
@@ -108,6 +135,13 @@ function is_for_platform(h::ObjectHandle, platform::AbstractPlatform)
             else
                 error("Unknown OS ABI type $(typeof(platform))")
             end
+        else
+            # If no OSABI, check whether it has a matching ELF note
+            if Sys.isfreebsd(platform)
+                if os_from_elf_note(h) != "FreeBSD"
+                    return false
+                end
+            end
         end
         # Check that the ELF arch matches our own
         m = h.header.e_machine

src/auditor/extra_checks.jl

@@ -5,7 +5,19 @@ using Dates: DateTime, datetime2unix
 
 function check_os_abi(oh::ObjectHandle, p::AbstractPlatform, rest...; verbose::Bool = false, kwargs...)
     if Sys.isfreebsd(p)
-        if oh.ei.osabi != ELF.ELFOSABI_FREEBSD
+        # On AArch64 and RISC-V, FreeBSD uses an ELF note section to identify itself rather
+        # than OS/ABI in the ELF header. In that case, the OS/ABI will be generic Unix (NONE).
+        # See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252490 and
+        # https://github.com/freebsd/freebsd-src/blob/main/lib/csu/common/crtbrand.S
+        if oh.ei.osabi == ELF.ELFOSABI_NONE
+            if os_from_elf_note(oh) != "FreeBSD"
+                if verbose
+                    @warn("$(basename(path(oh))) does not have a FreeBSD-branded ELF note " *
+                          "and may be unrecognized or unusable on $p")
+                end
+                return false
+            end
+        elseif oh.ei.osabi != ELF.ELFOSABI_FREEBSD
             # The dynamic loader should not have problems in this case, but the
             # linker may not appreciate.  Let the user know about this.
             if verbose
@@ -17,7 +29,8 @@ function check_os_abi(oh::ObjectHandle, p::AbstractPlatform, rest...; verbose::B
             end
             return false
         end
-    elseif call_abi(p) == "eabihf"
+    end
+    if call_abi(p) == "eabihf"
         # Make sure the object file has the hard-float ABI.  See Table 4-2 of
         # "ELF for the ARM Architecture" document
         # (https://developer.arm.com/documentation/ihi0044/e/).  Note: `0x000`