A parser and builder for the Linux process initial stack layout - use it to
decode or construct argc, argv, envp/envv, and auxv (auxiliary
vector).
Keywords: crt0, stack layout, AT values, AT pairs, auxvec, auxiliary vector
I use argv, envv, and auxv as names for the null-terminated arrays
(vectors) of the corresponding data. They have corresponding "data areas" where
pointers point to: argv data area, envp data area, and auxv data area.
Linux passes an initial stack layout to applications that contains argc,
argv, envp/envv, and the auxiliary vector. Normal applications don't
see this as the libc (crt0 component) abstracts this away. For more low-level
developers and kernel hackers creating and parsing this feature becomes
relevant.
This crate has been tested successfully by myself in a custom runtime system for a Microkernel which loads and starts unmodified Linux binaries. The Linux binary (the libc) was able to find all arguments, environment variables, and the data from the auxiliary vector. Everything was printed properly to stdout.
The following figure shows the technical details of the layout. The figure is taken from https://lwn.net/Articles/631631/.
------------------------------------------------------------- 0x7fff6c845000
0x7fff6c844ff8: 0x0000000000000000
_ 4fec: './stackdump\0' <------+
env / 4fe2: 'ENVVAR2=2\0' | <----+
\_ 4fd8: 'ENVVAR1=1\0' | <---+ |
/ 4fd4: 'two\0' | | | <----+
args | 4fd0: 'one\0' | | | <---+ |
\_ 4fcb: 'zero\0' | | | <--+ | |
3020: random gap padded to 16B boundary | | | | | |
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -| | | | | |
3019: 'x86_64\0' <-+ | | | | | |
auxv 3009: random data: ed99b6...2adcc7 | <-+ | | | | | |
data 3000: zero padding to align stack | | | | | | | |
. . . . . . . . . . . . . . . . . . . . . . . . . . .|. .|. .| | | | | |
2ff0: AT_NULL(0)=0 | | | | | | | |
2fe0: AT_PLATFORM(15)=0x7fff6c843019 --+ | | | | | | |
2fd0: AT_EXECFN(31)=0x7fff6c844fec ------|---+ | | | | |
2fc0: AT_RANDOM(25)=0x7fff6c843009 ------+ | | | | |
ELF 2fb0: AT_SECURE(23)=0 | | | | |
auxiliary 2fa0: AT_EGID(14)=1000 | | | | |
vector: 2f90: AT_GID(13)=1000 | | | | |
(id,val) 2f80: AT_EUID(12)=1000 | | | | |
pairs 2f70: AT_UID(11)=1000 | | | | |
2f60: AT_ENTRY(9)=0x4010c0 | | | | |
2f50: AT_FLAGS(8)=0 | | | | |
2f40: AT_BASE(7)=0x7ff6c1122000 | | | | |
2f30: AT_PHNUM(5)=9 | | | | |
2f20: AT_PHENT(4)=56 | | | | |
2f10: AT_PHDR(3)=0x400040 | | | | |
2f00: AT_CLKTCK(17)=100 | | | | |
2ef0: AT_PAGESZ(6)=4096 | | | | |
2ee0: AT_HWCAP(16)=0xbfebfbff | | | | |
2ed0: AT_SYSINFO_EHDR(33)=0x7fff6c86b000 | | | | |
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | | | | |
2ec8: environ[2]=(nil) | | | | |
2ec0: environ[1]=0x7fff6c844fe2 ------------------|-+ | | |
2eb8: environ[0]=0x7fff6c844fd8 ------------------+ | | |
2eb0: argv[3]=(nil) | | |
2ea8: argv[2]=0x7fff6c844fd4 ---------------------------|-|-+
2ea0: argv[1]=0x7fff6c844fd0 ---------------------------|-+
2e98: argv[0]=0x7fff6c844fcb ---------------------------+
0x7fff6c842e90: argc=3
Credits: https://lwn.net/Articles/631631/
This crate supports no_std-contexts plus allows construction of the data
structure for a different address space, i.e. the address space of a user
application.
✅ build data structure for current address space
✅ build data structure for different address space
✅ parse data structure for current address space + output referenced data/pointers
✅ parse data structure for different address space + prevent memory error / no dereferencing of pointers
1.85.0 stable
- https://lwn.net/Articles/631631/ (good overview with ASCII graphics)
- https://lwn.net/Articles/519085/
- https://elixir.bootlin.com/linux/v5.15.5/source/fs/binfmt_elf.c#L257 (code in Linux that constructs
auxv) - https://man7.org/linux/man-pages/man3/getauxval.3.html
- https://refspecs.linuxfoundation.org/ELF/zSeries/lzsabi0_zSeries/x895.html