std/sys/personality/dwarf/
eh.rs

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1//! Parsing of GCC-style Language-Specific Data Area (LSDA)
2//! For details see:
3//!  * <https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA/ehframechpt.html>
4//!  * <https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/dwarfext.html>
5//!  * <https://itanium-cxx-abi.github.io/cxx-abi/exceptions.pdf>
6//!  * <https://www.airs.com/blog/archives/460>
7//!  * <https://www.airs.com/blog/archives/464>
8//!
9//! A reference implementation may be found in the GCC source tree
10//! (`<root>/libgcc/unwind-c.c` as of this writing).
11
12#![allow(non_upper_case_globals)]
13#![allow(unused)]
14
15use core::ptr;
16
17use super::DwarfReader;
18
19pub const DW_EH_PE_omit: u8 = 0xFF;
20pub const DW_EH_PE_absptr: u8 = 0x00;
21
22pub const DW_EH_PE_uleb128: u8 = 0x01;
23pub const DW_EH_PE_udata2: u8 = 0x02;
24pub const DW_EH_PE_udata4: u8 = 0x03;
25pub const DW_EH_PE_udata8: u8 = 0x04;
26pub const DW_EH_PE_sleb128: u8 = 0x09;
27pub const DW_EH_PE_sdata2: u8 = 0x0A;
28pub const DW_EH_PE_sdata4: u8 = 0x0B;
29pub const DW_EH_PE_sdata8: u8 = 0x0C;
30
31pub const DW_EH_PE_pcrel: u8 = 0x10;
32pub const DW_EH_PE_textrel: u8 = 0x20;
33pub const DW_EH_PE_datarel: u8 = 0x30;
34pub const DW_EH_PE_funcrel: u8 = 0x40;
35pub const DW_EH_PE_aligned: u8 = 0x50;
36
37pub const DW_EH_PE_indirect: u8 = 0x80;
38
39#[derive(Copy, Clone)]
40pub struct EHContext<'a> {
41    pub ip: *const u8,                             // Current instruction pointer
42    pub func_start: *const u8,                     // Pointer to the current function
43    pub get_text_start: &'a dyn Fn() -> *const u8, // Get pointer to the code section
44    pub get_data_start: &'a dyn Fn() -> *const u8, // Get pointer to the data section
45}
46
47/// Landing pad.
48type LPad = *const u8;
49pub enum EHAction {
50    None,
51    Cleanup(LPad),
52    Catch(LPad),
53    Filter(LPad),
54    Terminate,
55}
56
57/// 32-bit ARM Darwin platforms uses SjLj exceptions.
58///
59/// The exception is watchOS armv7k (specifically that subarchitecture), which
60/// instead uses DWARF Call Frame Information (CFI) unwinding.
61///
62/// <https://github.com/llvm/llvm-project/blob/llvmorg-18.1.4/clang/lib/Driver/ToolChains/Darwin.cpp#L3107-L3119>
63pub const USING_SJLJ_EXCEPTIONS: bool =
64    cfg!(all(target_vendor = "apple", not(target_os = "watchos"), target_arch = "arm"));
65
66pub unsafe fn find_eh_action(lsda: *const u8, context: &EHContext<'_>) -> Result<EHAction, ()> {
67    if lsda.is_null() {
68        return Ok(EHAction::None);
69    }
70
71    let func_start = context.func_start;
72    let mut reader = DwarfReader::new(lsda);
73    let lpad_base = unsafe {
74        let start_encoding = reader.read::<u8>();
75        // base address for landing pad offsets
76        if start_encoding != DW_EH_PE_omit {
77            read_encoded_pointer(&mut reader, context, start_encoding)?
78        } else {
79            func_start
80        }
81    };
82    let call_site_encoding = unsafe {
83        let ttype_encoding = reader.read::<u8>();
84        if ttype_encoding != DW_EH_PE_omit {
85            // Rust doesn't analyze exception types, so we don't care about the type table
86            reader.read_uleb128();
87        }
88
89        reader.read::<u8>()
90    };
91    let action_table = unsafe {
92        let call_site_table_length = reader.read_uleb128();
93        reader.ptr.add(call_site_table_length as usize)
94    };
95    let ip = context.ip;
96
97    if !USING_SJLJ_EXCEPTIONS {
98        // read the callsite table
99        while reader.ptr < action_table {
100            unsafe {
101                // these are offsets rather than pointers;
102                let cs_start = read_encoded_offset(&mut reader, call_site_encoding)?;
103                let cs_len = read_encoded_offset(&mut reader, call_site_encoding)?;
104                let cs_lpad = read_encoded_offset(&mut reader, call_site_encoding)?;
105                let cs_action_entry = reader.read_uleb128();
106                // Callsite table is sorted by cs_start, so if we've passed the ip, we
107                // may stop searching.
108                if ip < func_start.wrapping_add(cs_start) {
109                    break;
110                }
111                if ip < func_start.wrapping_add(cs_start + cs_len) {
112                    if cs_lpad == 0 {
113                        return Ok(EHAction::None);
114                    } else {
115                        let lpad = lpad_base.wrapping_add(cs_lpad);
116                        return Ok(interpret_cs_action(action_table, cs_action_entry, lpad));
117                    }
118                }
119            }
120        }
121        // Ip is not present in the table. This indicates a nounwind call.
122        Ok(EHAction::Terminate)
123    } else {
124        // SjLj version:
125        // The "IP" is an index into the call-site table, with two exceptions:
126        // -1 means 'no-action', and 0 means 'terminate'.
127        match ip.addr() as isize {
128            -1 => return Ok(EHAction::None),
129            0 => return Ok(EHAction::Terminate),
130            _ => (),
131        }
132        let mut idx = ip.addr();
133        loop {
134            let cs_lpad = unsafe { reader.read_uleb128() };
135            let cs_action_entry = unsafe { reader.read_uleb128() };
136            idx -= 1;
137            if idx == 0 {
138                // Can never have null landing pad for sjlj -- that would have
139                // been indicated by a -1 call site index.
140                // FIXME(strict provenance)
141                let lpad = ptr::with_exposed_provenance((cs_lpad + 1) as usize);
142                return Ok(unsafe { interpret_cs_action(action_table, cs_action_entry, lpad) });
143            }
144        }
145    }
146}
147
148unsafe fn interpret_cs_action(
149    action_table: *const u8,
150    cs_action_entry: u64,
151    lpad: LPad,
152) -> EHAction {
153    if cs_action_entry == 0 {
154        // If cs_action_entry is 0 then this is a cleanup (Drop::drop). We run these
155        // for both Rust panics and foreign exceptions.
156        EHAction::Cleanup(lpad)
157    } else {
158        // If lpad != 0 and cs_action_entry != 0, we have to check ttype_index.
159        // If ttype_index == 0 under the condition, we take cleanup action.
160        let action_record = unsafe { action_table.offset(cs_action_entry as isize - 1) };
161        let mut action_reader = DwarfReader::new(action_record);
162        let ttype_index = unsafe { action_reader.read_sleb128() };
163        if ttype_index == 0 {
164            EHAction::Cleanup(lpad)
165        } else if ttype_index > 0 {
166            // Stop unwinding Rust panics at catch_unwind.
167            EHAction::Catch(lpad)
168        } else {
169            EHAction::Filter(lpad)
170        }
171    }
172}
173
174#[inline]
175fn round_up(unrounded: usize, align: usize) -> Result<usize, ()> {
176    if align.is_power_of_two() { Ok((unrounded + align - 1) & !(align - 1)) } else { Err(()) }
177}
178
179/// Reads an offset (`usize`) from `reader` whose encoding is described by `encoding`.
180///
181/// `encoding` must be a [DWARF Exception Header Encoding as described by the LSB spec][LSB-dwarf-ext].
182/// In addition the upper ("application") part must be zero.
183///
184/// # Errors
185/// Returns `Err` if `encoding`
186/// * is not a valid DWARF Exception Header Encoding,
187/// * is `DW_EH_PE_omit`, or
188/// * has a non-zero application part.
189///
190/// [LSB-dwarf-ext]: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/dwarfext.html
191unsafe fn read_encoded_offset(reader: &mut DwarfReader, encoding: u8) -> Result<usize, ()> {
192    if encoding == DW_EH_PE_omit || encoding & 0xF0 != 0 {
193        return Err(());
194    }
195    let result = unsafe {
196        match encoding & 0x0F {
197            // despite the name, LLVM also uses absptr for offsets instead of pointers
198            DW_EH_PE_absptr => reader.read::<usize>(),
199            DW_EH_PE_uleb128 => reader.read_uleb128() as usize,
200            DW_EH_PE_udata2 => reader.read::<u16>() as usize,
201            DW_EH_PE_udata4 => reader.read::<u32>() as usize,
202            DW_EH_PE_udata8 => reader.read::<u64>() as usize,
203            DW_EH_PE_sleb128 => reader.read_sleb128() as usize,
204            DW_EH_PE_sdata2 => reader.read::<i16>() as usize,
205            DW_EH_PE_sdata4 => reader.read::<i32>() as usize,
206            DW_EH_PE_sdata8 => reader.read::<i64>() as usize,
207            _ => return Err(()),
208        }
209    };
210    Ok(result)
211}
212
213/// Reads a pointer from `reader` whose encoding is described by `encoding`.
214///
215/// `encoding` must be a [DWARF Exception Header Encoding as described by the LSB spec][LSB-dwarf-ext].
216///
217/// # Errors
218/// Returns `Err` if `encoding`
219/// * is not a valid DWARF Exception Header Encoding,
220/// * is `DW_EH_PE_omit`, or
221/// * combines `DW_EH_PE_absptr` or `DW_EH_PE_aligned` application part with an integer encoding
222///   (not `DW_EH_PE_absptr`) in the value format part.
223///
224/// [LSB-dwarf-ext]: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/dwarfext.html
225unsafe fn read_encoded_pointer(
226    reader: &mut DwarfReader,
227    context: &EHContext<'_>,
228    encoding: u8,
229) -> Result<*const u8, ()> {
230    if encoding == DW_EH_PE_omit {
231        return Err(());
232    }
233
234    let base_ptr = match encoding & 0x70 {
235        DW_EH_PE_absptr => core::ptr::null(),
236        // relative to address of the encoded value, despite the name
237        DW_EH_PE_pcrel => reader.ptr,
238        DW_EH_PE_funcrel => {
239            if context.func_start.is_null() {
240                return Err(());
241            }
242            context.func_start
243        }
244        DW_EH_PE_textrel => (*context.get_text_start)(),
245        DW_EH_PE_datarel => (*context.get_data_start)(),
246        // aligned means the value is aligned to the size of a pointer
247        DW_EH_PE_aligned => {
248            reader.ptr = reader.ptr.with_addr(round_up(reader.ptr.addr(), size_of::<*const u8>())?);
249            core::ptr::null()
250        }
251        _ => return Err(()),
252    };
253
254    let mut ptr = if base_ptr.is_null() {
255        // any value encoding other than absptr would be nonsensical here;
256        // there would be no source of pointer provenance
257        if encoding & 0x0F != DW_EH_PE_absptr {
258            return Err(());
259        }
260        unsafe { reader.read::<*const u8>() }
261    } else {
262        let offset = unsafe { read_encoded_offset(reader, encoding & 0x0F)? };
263        base_ptr.wrapping_add(offset)
264    };
265
266    if encoding & DW_EH_PE_indirect != 0 {
267        ptr = unsafe { *(ptr.cast::<*const u8>()) };
268    }
269
270    Ok(ptr)
271}
std/sys/personality/dwarf/eh.rs

std/sys/personality/dwarf/
eh.rs
