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Implement the DT_GNU_HASH mechanism for vDSO parsing.
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Linux recently [removed] the `DT_HASH` section from the aarch64 vDSO. To
continue to be able to read vDSOs, implement the `DT_GNU_HASH` section,
following the logic in [this patch].

[removed]: torvalds/linux@48f6430
[this patch]: https://lkml.org/lkml/2024/12/6/828
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@sunfishcode
sunfishcode committed Jan 8, 2025
1 parent b6970cb commit 67e0afb
Showing 1 changed file with 118 additions and 42 deletions.
160 changes: 118 additions & 42 deletions src/backend/linux_raw/vdso.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,7 @@ pub(super) struct Vdso {
// Symbol table
symtab: *const Elf_Sym,
symstrings: *const u8,
gnu_hash: *const u32,
bucket: *const ElfHashEntry,
chain: *const ElfHashEntry,
nbucket: ElfHashEntry,
Expand All @@ -60,6 +61,16 @@ fn elf_hash(name: &CStr) -> u32 {
h
}

fn gnu_hash(name: &CStr) -> u32 {
let mut h: u32 = 5381;
for s in name.to_bytes() {
h = h
.wrapping_add(h.wrapping_mul(32))
.wrapping_add(u32::from(*s));
}
h
}

/// Create a `Vdso` value by parsing the vDSO at the `sysinfo_ehdr` address.
fn init_from_sysinfo_ehdr() -> Option<Vdso> {
// SAFETY: The auxv initialization code does extensive checks to ensure
Expand All @@ -80,6 +91,7 @@ fn init_from_sysinfo_ehdr() -> Option<Vdso> {
pv_offset: 0,
symtab: null(),
symstrings: null(),
gnu_hash: null(),
bucket: null(),
chain: null(),
nbucket: 0,
Expand Down Expand Up @@ -159,6 +171,11 @@ fn init_from_sysinfo_ehdr() -> Option<Vdso> {
)?
.as_ptr();
}
DT_GNU_HASH => {
vdso.gnu_hash =
check_raw_pointer::<u32>(vdso.addr_from_elf(d.d_un.d_ptr)? as *mut _)?
.as_ptr()
}
DT_VERSYM => {
vdso.versym =
check_raw_pointer::<u16>(vdso.addr_from_elf(d.d_un.d_ptr)? as *mut _)?
Expand All @@ -183,7 +200,10 @@ fn init_from_sysinfo_ehdr() -> Option<Vdso> {
// `check_raw_pointer` will have checked these pointers for null,
// however they could still be null if the expected dynamic table
// entries are absent.
if vdso.symstrings.is_null() || vdso.symtab.is_null() || hash.is_null() {
if vdso.symstrings.is_null()
|| vdso.symtab.is_null()
|| (hash.is_null() && vdso.gnu_hash.is_null())
{
return None; // Failed
}

Expand All @@ -192,10 +212,20 @@ fn init_from_sysinfo_ehdr() -> Option<Vdso> {
}

// Parse the hash table header.
vdso.nbucket = *hash.add(0);
//vdso.nchain = *hash.add(1);
vdso.bucket = hash.add(2);
vdso.chain = hash.add(vdso.nbucket as usize + 2);
if !vdso.gnu_hash.is_null() {
vdso.nbucket = *vdso.gnu_hash;
// The bucket array is located after the header (4 uint32) and the bloom
// filter (size_t array of gnu_hash[2] elements).
vdso.bucket = vdso
.gnu_hash
.add(4)
.add(size_of::<c::size_t>() / 4 * *vdso.gnu_hash.add(2) as usize);
} else {
vdso.nbucket = *hash.add(0);
//vdso.nchain = *hash.add(1);
vdso.bucket = hash.add(2);
vdso.chain = hash.add(vdso.nbucket as usize + 2);
}

// That's all we need.
Some(vdso)
Expand Down Expand Up @@ -261,49 +291,96 @@ impl Vdso {
&& (name == CStr::from_ptr(self.symstrings.add(aux.vda_name as usize).cast()))
}

/// Check to see if the symbol is the one we're looking for.
///
/// # Safety
///
/// The raw pointers inside `self` must be valid.
unsafe fn check_sym(
&self,
sym: &Elf_Sym,
i: u32,
name: &CStr,
version: &CStr,
ver_hash: u32,
) -> bool {
// Check for a defined global or weak function w/ right name.
//
// Accept `STT_NOTYPE` in addition to `STT_FUNC` for the symbol
// type, for compatibility with some versions of Linux on
// PowerPC64. See [this commit] in Linux for more background.
//
// [this commit]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/tools/testing/selftests/vDSO/parse_vdso.c?id=0161bd38c24312853ed5ae9a425a1c41c4ac674a
if ELF_ST_TYPE(sym.st_info) != STT_FUNC && ELF_ST_TYPE(sym.st_info) != STT_NOTYPE {
return false;
}
if ELF_ST_BIND(sym.st_info) != STB_GLOBAL && ELF_ST_BIND(sym.st_info) != STB_WEAK {
return false;
}
if name != CStr::from_ptr(self.symstrings.add(sym.st_name as usize).cast()) {
return false;
}

// Check symbol version.
if !self.versym.is_null()
&& !self.match_version(*self.versym.add(i as usize), version, ver_hash)
{
return false;
}

true
}

/// Look up a symbol in the vDSO.
pub(super) fn sym(&self, version: &CStr, name: &CStr) -> *mut c::c_void {
let ver_hash = elf_hash(version);
let name_hash = elf_hash(name);

// SAFETY: The pointers in `self` must be valid.
unsafe {
let mut chain = *self
.bucket
.add((ElfHashEntry::from(name_hash) % self.nbucket) as usize);

while chain != ElfHashEntry::from(STN_UNDEF) {
let sym = &*self.symtab.add(chain as usize);

// Check for a defined global or weak function w/ right name.
//
// Accept `STT_NOTYPE` in addition to `STT_FUNC` for the symbol
// type, for compatibility with some versions of Linux on
// PowerPC64. See [this commit] in Linux for more background.
//
// [this commit]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/tools/testing/selftests/vDSO/parse_vdso.c?id=0161bd38c24312853ed5ae9a425a1c41c4ac674a
if (ELF_ST_TYPE(sym.st_info) != STT_FUNC &&
ELF_ST_TYPE(sym.st_info) != STT_NOTYPE)
|| (ELF_ST_BIND(sym.st_info) != STB_GLOBAL
&& ELF_ST_BIND(sym.st_info) != STB_WEAK)
|| sym.st_shndx == SHN_UNDEF
|| sym.st_shndx == SHN_ABS
|| ELF_ST_VISIBILITY(sym.st_other) != STV_DEFAULT
|| (name != CStr::from_ptr(self.symstrings.add(sym.st_name as usize).cast()))
// Check symbol version.
|| (!self.versym.is_null()
&& !self.match_version(*self.versym.add(chain as usize), version, ver_hash))
{
chain = *self.chain.add(chain as usize);
continue;
}
if !self.gnu_hash.is_null() {
let mut h1: u32 = gnu_hash(name);

let sum = self.addr_from_elf(sym.st_value).unwrap();
assert!(
sum as usize >= self.load_addr as usize
&& sum as usize <= self.load_end as usize
);
return sum as *mut c::c_void;
let mut i = *self.bucket.add((h1 % self.nbucket) as usize);
if i == 0 {
return null_mut();
}
h1 |= 1;
let mut hashval = self
.bucket
.add(self.nbucket as usize)
.add((i - *self.gnu_hash.add(1)) as usize);
loop {
let sym: &Elf_Sym = &*self.symtab.add(i as usize);
let h2 = *hashval;
hashval = hashval.add(1);
if h1 == (h2 | 1) && self.check_sym(sym, i, name, version, ver_hash) {
let sum = self.addr_from_elf(sym.st_value).unwrap();
assert!(
sum as usize >= self.load_addr as usize
&& sum as usize <= self.load_end as usize
);
return sum as *mut c::c_void;
}
if (h2 & 1) != 0 {
break;
}
i += 1;
}
} else {
let mut i = *self.bucket.add((elf_hash(name) % self.nbucket) as usize);
while i != 0 {
let sym: &Elf_Sym = &*self.symtab.add(i as usize);
if sym.st_shndx != SHN_UNDEF && self.check_sym(sym, i, name, version, ver_hash)
{
let sum = self.addr_from_elf(sym.st_value).unwrap();
assert!(
sum as usize >= self.load_addr as usize
&& sum as usize <= self.load_end as usize
);
return sum as *mut c::c_void;
}
i = *self.chain.add(i as usize);
}
}
}

Expand All @@ -326,7 +403,6 @@ impl Vdso {

#[cfg(linux_raw)]
#[test]
#[ignore] // Until rustix is updated to the new vDSO format.
fn test_vdso() {
let vdso = Vdso::new().unwrap();
assert!(!vdso.symtab.is_null());
Expand Down

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