On Sat, Sep 17, 2022 at 06:31:20AM +0000, Christophe Leroy wrote: > > > Le 16/09/2022 à 21:40, Omar Sandoval a écrit : > > [Vous ne recevez pas souvent de courriers de osandov@xxxxxxxxxxx. D?couvrez pourquoi ceci est important ? https://aka.ms/LearnAboutSenderIdentification ] > > > > On Tue, Apr 28, 2020 at 06:21:05AM -0700, H.J. Lu wrote: > >> With the command-line option, -mx86-used-note=yes, the x86 assembler > >> in binutils 2.32 and above generates a program property note in a note > >> section, .note.gnu.property, to encode used x86 ISAs and features. But > >> kernel linker script only contains a single NOTE segment: > >> > >> PHDRS { > >> text PT_LOAD FLAGS(5); > >> data PT_LOAD FLAGS(6); > >> percpu PT_LOAD FLAGS(6); > >> init PT_LOAD FLAGS(7); > >> note PT_NOTE FLAGS(0); > >> } > >> SECTIONS > >> { > >> ... > >> .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not > >> e.*)) __stop_notes = .; } :text :note > >> ... > >> } > >> > >> The NOTE segment generated by kernel linker script is aligned to 4 bytes. > >> But .note.gnu.property section must be aligned to 8 bytes on x86-64 and > >> we get > >> > >> [hjl@gnu-skx-1 linux]$ readelf -n vmlinux > >> > >> Displaying notes found in: .notes > >> Owner Data size Description > >> Xen 0x00000006 Unknown note type: (0x00000006) > >> description data: 6c 69 6e 75 78 00 > >> Xen 0x00000004 Unknown note type: (0x00000007) > >> description data: 32 2e 36 00 > >> xen-3.0 0x00000005 Unknown note type: (0x006e6558) > >> description data: 08 00 00 00 03 > >> readelf: Warning: note with invalid namesz and/or descsz found at offset 0x50 > >> readelf: Warning: type: 0xffffffff, namesize: 0x006e6558, descsize: > >> 0x80000000, alignment: 8 > >> [hjl@gnu-skx-1 linux]$ > >> > >> Since note.gnu.property section in kernel image is never used, this patch > >> discards .note.gnu.property sections in kernel linker script by adding > >> > >> /DISCARD/ : { > >> *(.note.gnu.property) > >> } > >> > >> before kernel NOTE segment in generic NOTES. > >> > >> Signed-off-by: H.J. Lu <hjl.tools@xxxxxxxxx> > >> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > >> --- > >> include/asm-generic/vmlinux.lds.h | 7 +++++++ > >> 1 file changed, 7 insertions(+) > >> > >> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > >> index 71e387a5fe90..95cd678428f4 100644 > >> --- a/include/asm-generic/vmlinux.lds.h > >> +++ b/include/asm-generic/vmlinux.lds.h > >> @@ -833,7 +833,14 @@ > >> #define TRACEDATA > >> #endif > >> > >> +/* > >> + * Discard .note.gnu.property sections which are unused and have > >> + * different alignment requirement from kernel note sections. > >> + */ > >> #define NOTES \ > >> + /DISCARD/ : { \ > >> + *(.note.gnu.property) \ > >> + } \ > >> .notes : AT(ADDR(.notes) - LOAD_OFFSET) { \ > >> __start_notes = .; \ > >> KEEP(*(.note.*)) \ > >> -- > >> 2.25.4 > >> > > > > Hi, H.J., > > > > I recently ran into this same .notes corruption when building kernels on > > Arch Linux. > > > > What ended up happening to this patch? It doesn't appear to have been > > merged, and I couldn't find any further discussion about it. I'm happy > > to resend it for you if you need a hand. > > As far as I can see, ARM64 is doing something with that section, see > arch/arm64/include/asm/assembler.h > > Instead of discarding that section, would it be enough to force > alignment of .notes to 8 bytes ? > > Thanks > Christophe Unfortunately, "alignment requirement" here isn't just the starting alignment of the .notes section; it also refers to internal padding in the note metadata to keep things aligned. Changing this would break anyone who parses /sys/kernel/notes (e.g., perf). Here is a little more context around this mess: The System V gABI [1] says that the note header and descriptor should be aligned to 4 bytes for 32-bit files and 8 bytes for 64-bit files. However, Linux never followed this, and 4-byte alignment is used for both 32-bit and 64-bit files; see elf(5) [2]. The only exception as of 2022 is .note.gnu.property/NT_GNU_PROPERTY_TYPE_0, which is defined to follow the gABI alignment. There was a long thread discussing this back in 2018 with the subject "PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold" [3]. According to the gABI Linux Extensions [4], consumers are now supposed to use the p_align of the PT_NOTE segment instead of assuming an alignment. There are a few issues with this for the kernel: * The vmlinux linker script squishes together all of the notes sections with different alignments without adjusting their internal padding, but sets p_align to the maximum required alignment. This is what confuses readelf and co: they expect 8-byte alignment, but most of the note entries are only padded for 4-byte alignment. * The vmlinux .notes section is exported as /sys/kernel/notes. This is stable ABI and has always had 4-byte alignment; all existing parsers assume this. * /sys/kernel/notes doesn't currently have a way to specify an alternate alignment anyways. My suggestion would be to keep .note.gnu.property in its own section in vmlinux, or create a new .notes8 section with 8-byte alignment, and leave .notes and /sys/kernel/notes alone. I'm not sure what exactly arch/arm64/include/asm/assembler.h is doing with this file. Perhaps the author, Mark Brown, can clarify? 1: http://www.sco.com/developers/gabi/latest/ch5.pheader.html#note_section 2: https://man7.org/linux/man-pages/man5/elf.5.html#:~:text=Notes%20(Nhdr) 3: https://public-inbox.org/libc-alpha/13a92cb0-a993-f684-9a96-e02e4afb1bef@xxxxxxxxxx/ 4: https://gitlab.com/x86-psABIs/Linux-ABI
