amd64: initialize td_frame stack area for init(8) main thread

[kib]

Summary

Unitialized td_frame mostly does not matter since all registers are
overwritten on exec to activate init(8).  Except PSL_T bit from the
%rflags which might leak into fresh init as garbage, causing spurious
SIGTRAPs delivered to init until first syscall is executed.

[emaste]

Would it be excessive to just zero td_frame in MI code?

[kib]

In D44498#1014879, @emaste wrote:

Would it be excessive to just zero td_frame in MI code?

I am not sure that this is the correct state of the registers file for all arches.

[jhibbits]

In D44498#1014883, @kib wrote:

In D44498#1014879, @emaste wrote:

Would it be excessive to just zero td_frame in MI code?

I am not sure that this is the correct state of the registers file for all arches.

Does it not go through exec_setregs() at some point through kern_execve()? That should set all the registers.

[kib]

In D44498#1014892, @jhibbits wrote:

In D44498#1014883, @kib wrote:

In D44498#1014879, @emaste wrote:

Would it be excessive to just zero td_frame in MI code?

I am not sure that this is the correct state of the registers file for all arches.

Does it not go through exec_setregs() at some point through kern_execve()? That should set all the registers.

Please read the summary which specifically mentions %rflags.PSL_T as the cause of this patch.

[jhibbits]

In D44498#1014893, @kib wrote:

In D44498#1014892, @jhibbits wrote:

In D44498#1014883, @kib wrote:

In D44498#1014879, @emaste wrote:

Would it be excessive to just zero td_frame in MI code?

I am not sure that this is the correct state of the registers file for all arches.

Does it not go through exec_setregs() at some point through kern_execve()? That should set all the registers.

Please read the summary which specifically mentions %rflags.PSL_T as the cause of this patch.

Sorry, I know nothing about amd64 CPU registers. How is that different from other registers? I would expect all architected per-process registers to be set up in exec_setregs().

Speaking from the PowerPC side, exec_setregs() already bzero()s the trap frame anyway.

Ah, reading exec_setregs() for amd64, I see that it preserves the initial PSL_T. Got it now.

Going back to @emaste's query, it seems to me that bzeroing in MI makes the most sense. IMO exec_setregs(), and the entire MD code path from kern_execve() should assume zero as a 'normal' state, and init should be a 'normal' process from exec standpoint.

[kib]

Do memset() in start_init() unconditionally