M-Labs/nac3
M-Labs/nac3
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ld: Support multiple CFIs with different encoding in .eh_frame

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We now parse each CFI to read its encoding as opposed to assuming that
all CFIs within the same EH_Frame uses the same encoding. FDEs are now
iterated in a per-CFI manner.
This commit is contained in:
David Mak 2023-10-19 15:27:52 +08:00
parent aa92778363
commit e49b760e34
2 changed files with 194 additions and 71 deletions
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250
nac3ld/src/dwarf.rs
View File

@ -27,20 +27,33 @@ pub const DW_EH_PE_indirect: u8 = 0x80;
pub struct DwarfReader<'a> {
pub slice: &'a [u8],
pub virt_addr: u32,
base_slice: &'a [u8],
base_virt_addr: u32,
}
impl<'a> DwarfReader<'a> {
pub fn new(slice: &[u8], virt_addr: u32) -> DwarfReader {
DwarfReader { slice, virt_addr }
DwarfReader { slice, virt_addr, base_slice: slice, base_virt_addr: virt_addr }
}
pub fn offset(&mut self, offset: i32) {
/// Creates a new instance from another instance of [DwarfReader], optionally removing any
/// offsets previously applied to the other instance.
pub fn from_reader(other: &DwarfReader<'a>, reset_offset: bool) -> DwarfReader<'a> {
if reset_offset {
DwarfReader::new(&other.base_slice, other.base_virt_addr)
} else {
DwarfReader::new(&other.slice, other.virt_addr)
}
}
pub fn offset(&mut self, offset: u32) {
self.slice = &self.slice[offset as usize..];
self.virt_addr = self.virt_addr.wrapping_add(offset as u32);
self.virt_addr = self.virt_addr.wrapping_add(offset);
}
// ULEB128 and SLEB128 encodings are defined in Section 7.6 - "Variable
// Length Data".
/// ULEB128 and SLEB128 encodings are defined in Section 7.6 - "Variable Length Data" of the
/// [DWARF-4 Manual](https://dwarfstd.org/doc/DWARF4.pdf).
pub fn read_uleb128(&mut self) -> u64 {
let mut shift: usize = 0;
let mut result: u64 = 0;
@ -70,7 +83,7 @@ impl<'a> DwarfReader<'a> {
}
// sign-extend
if shift < u64::BITS && (byte & 0x40) != 0 {
result |= (!0 as u64) << shift;
result |= (!0u64) << shift;
}
result as i64
}
@ -200,38 +213,69 @@ fn round_up(unrounded: usize, align: usize) -> Result<usize, ()> {
}
}
// Minimalistic structure to store everything needed for parsing FDEs to synthesize
// .eh_frame_hdr section. Since we are only linking 1 object file, there should only be 1 call
// frame information (CFI) record, so there should be only 1 common information entry (CIE).
// So the class parses the only CIE on init, cache the encoding info, then parse the FDE on
// iterations based on the cached encoding format.
/// Minimalistic structure to store everything needed for parsing FDEs to synthesize `.eh_frame_hdr`
/// section.
///
/// Refer to [The Linux Standard Base Core Specification, Generic Part](https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html)
/// for more information.
pub struct EH_Frame<'a> {
// It refers to the augmentation data that corresponds to 'R' in the augmentation string
pub fde_pointer_encoding: u8,
pub fde_reader: DwarfReader<'a>,
pub fde_sz: usize,
reader: DwarfReader<'a>,
}
impl<'a> EH_Frame<'a> {
/// Creates an [EH_Frame] using the bytes in the `.eh_frame` section and its address in the ELF
/// file.
pub fn new(eh_frame_slice: &[u8], eh_frame_addr: u32) -> Result<EH_Frame, ()> {
let mut cie_reader = DwarfReader::new(eh_frame_slice, eh_frame_addr);
let eh_frame_size = eh_frame_slice.len();
Ok(EH_Frame { reader: DwarfReader::new(eh_frame_slice, eh_frame_addr) })
}
/// Returns an [Iterator] over all Call Frame Information (CFI) records.
pub fn cfi_records(&self) -> CFI_Records<'a> {
let reader = DwarfReader::from_reader(&self.reader, true);
let len = reader.slice.len();
CFI_Records {
reader,
available: len,
}
}
}
/// A single Call Frame Information (CFI) record.
///
/// From the [specification](https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html):
///
/// > Each CFI record contains a Common Information Entry (CIE) record followed by 1 or more Frame
/// Description Entry (FDE) records.
pub struct CFI_Record<'a> {
// It refers to the augmentation data that corresponds to 'R' in the augmentation string
fde_pointer_encoding: u8,
fde_reader: DwarfReader<'a>,
}
impl<'a> CFI_Record<'a> {
pub fn from_reader(cie_reader: &mut DwarfReader<'a>) -> Result<CFI_Record<'a>, ()> {
let length = cie_reader.read_u32();
let fde_reader = match length {
// eh_frame with 0 lengths means the CIE is terminated
// while length == u32::MAX means that the length is only representable with 64 bits,
0 => panic!("Cannot create an EH_Frame from a termination CIE"),
// length == u32::MAX means that the length is only representable with 64 bits,
// which does not make sense in a system with 32-bit address.
0 | 0xFFFFFFFF => unimplemented!(),
0xFFFFFFFF => unimplemented!(),
_ => {
let mut fde_reader = DwarfReader::new(cie_reader.slice, cie_reader.virt_addr);
fde_reader.offset(length as i32);
let mut fde_reader = DwarfReader::from_reader(&cie_reader, false);
fde_reader.offset(length);
fde_reader
}
};
let fde_sz = eh_frame_size - mem::size_of::<u32>() - length as usize;
// Routine check on the .eh_frame well-formness, in terms of CIE ID & Version args.
assert_eq!(cie_reader.read_u32(), 0);
let cie_ptr = cie_reader.read_u32();
assert_eq!(cie_ptr, 0);
assert_eq!(cie_reader.read_u8(), 1);
// Parse augmentation string
@ -242,7 +286,7 @@ impl<'a> EH_Frame<'a> {
// Skip code/data alignment factors & return address register along the way as well
// We only tackle the case where 'z' and 'R' are part of the augmentation string, otherwise
// we cannot get the addresses to make .eh_frame_hdr
let mut aug_data_reader = DwarfReader::new(cie_reader.slice, cie_reader.virt_addr);
let mut aug_data_reader = DwarfReader::from_reader(&cie_reader, false);
let mut aug_str_len = 0;
loop {
if aug_data_reader.read_u8() == b'\0' {
@ -279,44 +323,121 @@ impl<'a> EH_Frame<'a> {
}
assert_ne!(fde_pointer_encoding, DW_EH_PE_omit);
Ok(EH_Frame { fde_pointer_encoding, fde_reader, fde_sz })
Ok(CFI_Record {
fde_pointer_encoding,
fde_reader,
})
}
pub fn iterate_fde(&self, callback: &mut dyn FnMut(u32, u32)) -> Result<(), ()> {
/// Returns a [DwarfReader] initialized to the first Frame Description Entry (FDE) of this CFI
/// record.
pub fn get_fde_reader(&self) -> DwarfReader<'a> {
DwarfReader::from_reader(&self.fde_reader, true)
}
/// Returns an [Iterator] over all Frame Description Entries (FDEs).
pub fn fde_records(&self) -> FDE_Records<'a> {
let reader = self.get_fde_reader();
let len = reader.slice.len();
FDE_Records {
pointer_encoding: self.fde_pointer_encoding,
reader,
available: len,
}
}
}
/// [Iterator] over Call Frame Information (CFI) records in an
/// [Exception Handling (EH) frame][EH_Frame].
pub struct CFI_Records<'a> {
reader: DwarfReader<'a>,
available: usize,
}
impl<'a> Iterator for CFI_Records<'a> {
type Item = CFI_Record<'a>;
fn next(&mut self) -> Option<Self::Item> {
loop {
if self.available == 0 {
return None;
}
let mut this_reader = DwarfReader::from_reader(&self.reader, false);
// Remove the length of the header and the content from the counter
let length = self.reader.read_u32();
let length = match length {
// eh_frame with 0-length means the CIE is terminated
0 => return None,
0xFFFFFFFF => unimplemented!("CIE entries larger than 4 bytes not supported"),
other => other,
} as usize;
// Remove the length of the header and the content from the counter
self.available -= length + mem::size_of::<u32>();
let mut next_reader = DwarfReader::from_reader(&self.reader, false);
next_reader.offset(length as u32);
let cie_ptr = self.reader.read_u32();
self.reader = next_reader;
// Skip this record if it is a FDE
if cie_ptr == 0 {
// Rewind back to the start of the CFI Record
return Some(CFI_Record::from_reader(&mut this_reader).ok().unwrap())
}
}
}
}
/// [Iterator] over Frame Description Entries (FDEs) in an
/// [Exception Handling (EH) frame][EH_Frame].
pub struct FDE_Records<'a> {
pointer_encoding: u8,
reader: DwarfReader<'a>,
available: usize,
}
impl<'a> Iterator for FDE_Records<'a> {
type Item = (u32, u32);
fn next(&mut self) -> Option<Self::Item> {
// Parse each FDE to obtain the starting address that the FDE applies to
// Send the FDE offset and the mentioned address to a callback that write up the
// .eh_frame_hdr section
let mut remaining_len = self.fde_sz;
let mut reader = DwarfReader::new(self.fde_reader.slice, self.fde_reader.virt_addr);
loop {
if remaining_len == 0 {
break;
}
let fde_virt_addr = reader.virt_addr;
let length = match reader.read_u32() {
0 | 0xFFFFFFFF => unimplemented!(),
other => other,
};
if self.available == 0 {
return None;
}
// Remove the length of the header and the content from the counter
remaining_len -= length as usize + mem::size_of::<u32>();
let mut next_fde_reader = DwarfReader::new(reader.slice, reader.virt_addr);
next_fde_reader.offset(length as i32);
let length = match self.reader.read_u32() {
// eh_frame with 0-length means the CIE is terminated
0 => return None,
0xFFFFFFFF => unimplemented!("CIE entries larger than 4 bytes not supported"),
other => other,
} as usize;
// Only parse FDEs, indicated by its CIE pointer being non-zero
let cie_ptr = reader.read_u32();
if cie_ptr != 0 {
// Parse PC Begin using the encoding scheme mentioned in the CIE
let pc_begin = read_encoded_pointer_with_pc(&mut reader, self.fde_pointer_encoding)?;
// Remove the length of the header and the content from the counter
self.available -= length + mem::size_of::<u32>();
let mut next_fde_reader = DwarfReader::from_reader(&self.reader, false);
next_fde_reader.offset(length as u32);
callback(pc_begin as u32, fde_virt_addr);
}
let cie_ptr = self.reader.read_u32();
let next_val = if cie_ptr != 0 {
let pc_begin = read_encoded_pointer_with_pc(&mut self.reader, self.pointer_encoding)
.expect("Failed to read PC Begin");
Some((pc_begin as u32, self.reader.virt_addr))
} else {
None
};
reader = next_fde_reader;
}
self.reader = next_fde_reader;
Ok(())
next_val
}
}
@ -327,29 +448,32 @@ pub struct EH_Frame_Hdr<'a> {
}
impl<'a> EH_Frame_Hdr<'a> {
// Create a EH_Frame_Hdr object, and write out the fixed fields of .eh_frame_hdr to memory
// eh_frame_ptr_enc will be 0x1B (PC-relative, 4 bytes)
// table_enc will be 0x3B (Relative to the start of .eh_frame_hdr, 4 bytes)
// Load address is not known at this point.
/// Create a [EH_Frame_Hdr] object, and write out the fixed fields of `.eh_frame_hdr` to memory.
///
/// Load address is not known at this point.
pub fn new(
eh_frame_hdr_slice: &mut [u8],
eh_frame_hdr_addr: u32,
eh_frame_addr: u32,
) -> EH_Frame_Hdr {
let mut writer = DwarfWriter::new(eh_frame_hdr_slice);
writer.write_u8(1);
writer.write_u8(0x1B);
writer.write_u8(0x03);
writer.write_u8(0x3B);
let eh_frame_offset =
(eh_frame_addr).wrapping_sub(eh_frame_hdr_addr + ((mem::size_of::<u8>() as u32) * 4));
writer.write_u32(eh_frame_offset);
writer.write_u32(0);
writer.write_u8(1); // version
writer.write_u8(0x1B); // eh_frame_ptr_enc - PC-relative 4-byte signed value
writer.write_u8(0x03); // fde_count_enc - 4-byte unsigned value
writer.write_u8(0x3B); // table_enc - .eh_frame_hdr section-relative 4-byte signed value
let eh_frame_offset = eh_frame_addr
.wrapping_sub(eh_frame_hdr_addr + writer.offset as u32 + ((mem::size_of::<u8>() as u32) * 4));
writer.write_u32(eh_frame_offset); // eh_frame_ptr
writer.write_u32(0); // `fde_count`, will be written in finalize_fde
EH_Frame_Hdr { fde_writer: writer, eh_frame_hdr_addr, fdes: Vec::new() }
}
/// The offset of the `fde_count` value relative to the start of the `.eh_frame_hdr` section in
/// bytes.
fn fde_count_offset() -> usize {
8
}
@ -391,7 +515,7 @@ impl<'a> EH_Frame_Hdr<'a> {
fde_count += 1;
}
reader.offset(entry_length as i32 - mem::size_of::<u32>() as i32)
reader.offset(entry_length - mem::size_of::<u32>() as u32)
}
12 + fde_count * 8

7
nac3ld/src/lib.rs
View File

@ -556,10 +556,9 @@ impl<'a> Linker<'a> {
eh_frame_hdr_rec.shdr.sh_offset,
eh_frame_rec.shdr.sh_offset,
);
let mut fde_callback = |init_pos, virt_addr| eh_frame_hdr.add_fde(init_pos, virt_addr);
eh_frame
.iterate_fde(&mut fde_callback)
.map_err(|()| "failed to add FDE to .eh_frame_hdr while iterating .eh_frame")?;
eh_frame.cfi_records()
.flat_map(|cfi| cfi.fde_records())
.for_each(&mut |(init_pos, virt_addr)| eh_frame_hdr.add_fde(init_pos, virt_addr));
// Sort FDE entries in .eh_frame_hdr
eh_frame_hdr.finalize_fde();