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| author | Fangrui Song <maskray@google.com> | 2023-12-20 11:47:37 -0800 |
|---|---|---|
| committer | Copybara-Service <copybara-worker@google.com> | 2023-12-20 11:48:40 -0800 |
| commit | 7a1898a04fb3ab83b869e7967ddd7721696490a3 (patch) | |
| tree | 897bf0fc86b71f17e42603c11c0c80b5c232128a | |
| parent | 299dbc588e4d2a6e23a4f99d5d224c042ab4a9a5 (diff) | |
| download | abseil-cpp-7a1898a04fb3ab83b869e7967ddd7721696490a3.tar.gz | |
AddressIsReadable: improve comments
Linux kernel's rt_sigprocmask correctly handles an unaligned user address[1].
The original issue was for qemu-user, which seems long irrelevant.
Tested locally on an AArch64 CPU and qemu-aarch64-static.
The alignment operation actually serves another purpose: when addr resides in
the last 7 bytes of a page (unaligned), check only the current page and not the
next. Update the comment.
[1]:
kernel/signal.c `SYSCALL_DEFINE4(rt_sigprocmask`
arch/arm64/include/asm/uaccess.h:raw_copy_from_user
arch/arm64/lib/copy_template.S "alignment handled by the hardware"
PiperOrigin-RevId: 592618320
Change-Id: Ifbd05aba42f46e36e710cca940570213036b3ce0
| -rw-r--r-- | absl/debugging/internal/address_is_readable.cc | 6 |
1 files changed, 4 insertions, 2 deletions
diff --git a/absl/debugging/internal/address_is_readable.cc b/absl/debugging/internal/address_is_readable.cc index 91eaa76f..be17a105 100644 --- a/absl/debugging/internal/address_is_readable.cc +++ b/absl/debugging/internal/address_is_readable.cc @@ -50,8 +50,10 @@ namespace debugging_internal { // NOTE: any new system calls here may also require sandbox reconfiguration. // bool AddressIsReadable(const void *addr) { - // Align address on 8-byte boundary. On aarch64, checking last - // byte before inaccessible page returned unexpected EFAULT. + // rt_sigprocmask below checks 8 contiguous bytes. If addr resides in the + // last 7 bytes of a page (unaligned), rt_sigprocmask would additionally + // check the readability of the next page, which is not desired. Align + // address on 8-byte boundary to check only the current page. const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~uintptr_t{7}; addr = reinterpret_cast<const void *>(u_addr); |