* [Bug tapsets/26015] New: make syscall arguments symbol-writeable again
@ 2020-05-20 11:37 fche at redhat dot com
2020-06-17 18:57 ` [Bug tapsets/26015] " sapatel at redhat dot com
0 siblings, 1 reply; 2+ messages in thread
From: fche at redhat dot com @ 2020-05-20 11:37 UTC (permalink / raw)
To: systemtap
https://sourceware.org/bugzilla/show_bug.cgi?id=26015
Bug ID: 26015
Summary: make syscall arguments symbol-writeable again
Product: systemtap
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: tapsets
Assignee: systemtap at sourceware dot org
Reporter: fche at redhat dot com
Blocks: 25580
Target Milestone: ---
With the linux 4.17+ transition to pt_regs passing syscall wrappers (the
nd2_syscall.* family of probe point aliases), as well as the tracepoint
fallbacks (the tp_syscall.* family), we've lost the ability to modify syscall
parameters, even in guru mode. That's because the parameters are copied out of
pt_regs into script level variables, but that's a one-way trip only.
IOW, previously we could do this, because the syscall alias expanded to a dwarf
probe of the low level syscall handler function:
stap -g -e 'probe syscall.foo { if (uid() == 0) $var = 2 }'
but now there is no $var.
One way might be to extend the nd2_* and tp_* suite with a construct using
epilogue-style probe aliases to copy back modified values into the pt_regs.
This might almost work:
old:
probe nd2_syscall.mknod = kprobe.function(@arch_syscall_prefix "sys_mknod") ?
{ // ....
_SYSCALL_MKNOD_REGARGS
// ....
}
@define _SYSCALL_MKNOD_REGARGS
%(
pathname = user_string_quoted(pointer_arg(1))
mode = uint_arg(2)
mode_str = _mknod_mode_str(mode)
dev = uint_arg(3)
%)
add:
probe nd2_syscall.mknod += kprobe.function(@arch_syscall_prefix "sys_mknod") ?
{ // ...
_SYSCALL_MKNOD_REGARGS_STORE
}
@define _SYSCALL_MKNOD_REGARGS_STORE
%(
store_user_string(pointer_arg(1), pathname) // or punt
set_uint_arg(2, mode)
set_uint_arg(3, dev)
// ...
%)
then
stap -g -e 'probe syscall.mknod { if (uid() == 0) mode &= 0700 }'
would work (to make root mknod's always have umask 077, apprx.).
Or we may be able to expose the $mode name as an rvalue & lvalue, via some sort
of newfangled macro trickery? Maybe let probe points include a list of
simulated context variables in context, with getter/setter expansion
expressions kind of like tree rewriting rules:
probe nd2_syscall.mknod = kprobe.function(@arch_syscall_prefix "sys_mknod") ?
[$mode, get_uint_arg(2), set_uint_arg(2,$mode)]
{
mode = $mode
}
where then the $mode symbol itself becomes read-write.
Referenced Bugs:
https://sourceware.org/bugzilla/show_bug.cgi?id=25580
[Bug 25580] lp tracker
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2020-05-20 11:37 [Bug tapsets/26015] New: make syscall arguments symbol-writeable again fche at redhat dot com
2020-06-17 18:57 ` [Bug tapsets/26015] " sapatel at redhat dot com
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