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manage: fix emergency buffer and north road calculation #993

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Merged
pkova merged 3 commits into from
May 6, 2026
+8 −7
Merged

manage: fix emergency buffer and north road calculation #993

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1848a7b
manage: fix emergency buffer and north road calculation
pkova Apr 1, 2026
f3c066e
melt: make sure emergency buffer is never structurally shared
pkova May 6, 2026
9e4ead9
Merge branch 'develop' into pkova/meme-fiz
pkova May 6, 2026
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2 changes: 1 addition & 1 deletion pkg/noun/allocate.h
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Original file line number Diff line number Diff line change
Expand Up @@ -286,7 +286,7 @@ STATIC_ASSERT( u3a_vits <= u3a_min_log,

/* u3a_is_north(): yes if road [r] is north road.
*/
# define u3a_is_north(r) __((r)->cap_p > (r)->hat_p)
# define u3a_is_north(r) __((r)->mat_p > (r)->rut_p)
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@joemfb joemfb Apr 1, 2026

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A road is north if the heap is low and the stack is high; south is opposite. This check has always been implemented using the innermost bounds of the stack ("cap") and heap ("hat").

The heap growing over the stack is guarded by explicit checks in the allocator, but the stack growing over the heap is not (the "cap" is constantly moving, so such checks have a measurable cost). Instead, a PROT_NONE guard page floats between the heap and the stack. Stores into it are caught by a global SIGSEGV handler and the page is repositioned if possible. If not the fault is handled with u3m_signal(c3__meme). In that state, the old implementation of u3a_is_north() will give the wrong answer, which wreaks havoc on refcounting and de/allocation logic.

This change makes u3a_is_north() robust against overflow states, by comparing the immutable outermost bounds of the heap ("rut") and stack ("mat").


/* u3a_is_south(): yes if road [r] is south road.
*/
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9 changes: 3 additions & 6 deletions pkg/noun/manage.c
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Expand Up @@ -398,9 +398,7 @@ _cm_signal_deep(void)
// go utterly haywire.
//
if ( 0 == u3H->rod_u.bug.mer ) {
u3H->rod_u.bug.mer = u3i_string(
"emergency buffer with sufficient space to cons the trace and bail"
);
u3H->rod_u.bug.mer = u3i_tape("emergency buffer");
}

u3t_boot();
Expand Down Expand Up @@ -1217,13 +1215,12 @@ u3m_hate(c3_w pad_w)
c3_w fag_w = u3R->how.fag_w;
u3m_leap(pad_w);

u3R->bug.mer = u3i_tape("emergency buffer");

// inherit forward-flowing flags
//
u3R->how.fag_w |= (fag_w & u3a_flag_cash);

u3R->bug.mer = u3i_string(
"emergency buffer with sufficient space to cons the trace and bail"
);
}

// RETAINS `now`.
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4 changes: 4 additions & 0 deletions pkg/vere/melt.c
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Expand Up @@ -165,6 +165,8 @@ u3_melt_all(FILE *fil_u)
//
u3_assert( &(u3H->rod_u) == u3R );

u3z(u3H->rod_u.bug.mer);

can_u.siz_w = 32;
can_u.tac = c3_malloc(sizeof(*can_u.tac) * can_u.siz_w);

Expand Down Expand Up @@ -194,6 +196,8 @@ u3_melt_all(FILE *fil_u)

c3_free(can_u.tac);

u3H->rod_u.bug.mer = u3i_tape("emergency buffer");

return u3a_idle(u3R) - pre_w;
}

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