Solved truss question about how strings are displayed

D

dmoran85

Hello,

I'm recently arrived to FreeBSD from too many years working with Linux and I'm trying to put the things right with my new FreeBSD :).

A very good tool for me was strace, because you can understand what are happening behind the scenes, so I decided to use truss to understand how FreeBSD works (probably dtrace is the best way but I'm going step by step). Using truss I found something annoying (for me):

Code: 
$ truss head -1 /etc/passwd 2>&1 | grep open
openat(0xffffff9c,0x800622000,0x100000,0x0,0xfffffffffffffc00,0x400308) = 3 (0x3)
openat(0xffffff9c,0x800618703,0x100000,0x0,0xfffffffffffffc00,0x0) = 3 (0x3)
openat(0xffffff9c,0x80061f060,0x100000,0x0,0x101010101010101,0x8080808080808080) = 3 (0x3)
open("/etc/passwd",O_RDONLY,0666)  = 3 (0x3)
$

Is there any reason which truss does not display openat strings arguments? It's impossible that nobody have realized about it because it's hard to find a process not executing openat, so I supposed there is a good reason.
 
I would guess the implementation is such that the information about the argument types for system calls is not available so truss(1) sees the second argument to openat(2) as a number but it doesn't know that it's in fact a pointer, char *.

The manual page says that it's modeled after similar tools from System V and SunOS so I'd guess those operated the same.
 
Someone obviously thought about implementing such features. The -f option turns on string expansion for the execve(2) system call but only for that one system call:

Code: 
freebsd10 ~ % truss -af env head -1 /etc/passwd 2>&1 | grep exec
8592: execve("/sbin/head", [ "head", "-1", "/etc/passwd" ],<missing argument>) ERR#2 'No such file or directory'
8592: execve("/bin/head", [ "head", "-1", "/etc/passwd" ],<missing argument>) ERR#2 'No such file or directory'
8592: execve("/usr/sbin/head", [ "head", "-1", "/etc/passwd" ],<missing argument>) ERR#2 'No such file or directory'
8592: execve("/usr/bin/head", [ "head", "-1", "/etc/passwd" ],<missing argument>) = 0 (0x0)
freebsd10 ~ %
 
It has it's own table
Code: 
$ grep open /usr/src/usr.bin/truss/syscalls.c
  { .name = "open", .ret_type = 1, .nargs = 3,
  { .name = "linux_open", .ret_type = 1, .nargs = 3,
  { .name = "posix_openpt", .ret_type = 1, .nargs = 1,

Juha
 
Yep,

Adding this
Code: 
  { .name = "openat", .ret_type = 1, .nargs = 4,
  .args = { {Int, 0}, { Name | IN, 1 } , { Open, 2 }, { Octal, 3 } } },

Works fine:

Code: 
$ truss head -1 /etc/passwd 2>&1 | grep open
openat(-100,"/etc/libmap.conf",O_CLOEXEC,00)  = 3 (0x3)
openat(-100,"/var/run/ld-elf.so.hints",O_CLOEXEC,00) = 3 (0x3)
openat(-100,"/lib/libc.so.7",O_CLOEXEC,00)  = 3 (0x3)
open("/etc/passwd",O_RDONLY,0666)  = 3 (0x3)
$
 
In FreeBSD-CURRENT it's already added to the table as:

Code: 
  { .name = "openat", .ret_type = 1, .nargs = 4,
  .args = { { Atfd, 0 }, { Name | IN, 1 }, { Open, 2 },
  { Octal, 3 } } },
 
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